How can I get the mugs in the top basket clean

Try rotating the arm manually. Check whether its freely rotating. Clean the rotating axis area.

Check whether the pressure reaching the top spray arm hose.

your top spray arm is blocked and not rotating,remove it from top basket and thoroughly clean it paying particular attention to the holes right at the ends these are angled to drive the arms round if they block up the arms will not rotate.

Please post the model number of your dishwasher here.

I am not familiar with the HorPoint Aquarius dishwasher, but if it is a newer, water conserving, energy saving model, it ~might~ have a check valve that switches/alternates the water from the lower to the upper rack (Other manufactures do this, including LG.)

I know you think I am nuts, but your problem ~may~ not be the wash arm. It really sounds like your heating element is not working, which then does not get the detergent powder up to the high temp it needs to activate the cleaning ability.

Also, buy a box of name brand detergent powder and use it rather that an ultra-cheap box. It really does make a difference with dishwasher detergent.

Good Luck!

Please post back here with what you find, OK?

Our Bosch has a round knob in the center of the arm, under the basket. ROtate it 90 degrees, and the arm drops away and can be cleaned.

The problem likely is the cutter/chopper filter.
1. Remove bottom basket. Remove top basket (Pop off 'keepers' at the ends of the slidding shelf supports).
2. Remove bottom washing arm. Unscrew (Gently but firmily) centre post. Lift arm assembly out.
3. Remove overhead conduit. One screw at back and one at top (Philips #2 screwdriver bit). Rotate whole assembly 45 deg to the right (Clockwise) and lift off of centre post It can then be taken apart to clean.
4. Remove centre post (Pull straight up) Remove 4 screws (Star T-20 screwdriver bit). Lift whole plastic assembly out. Pop off the screen (Flat blade screwdriver around the edge) and clean.
5. Remove cap over cutter/chopper. There is a single screw at left front (Under the just removed centre assembly). Remove (Star T-15 screwdriver bit) screw and cap (Needs a bit of wiggling to get it free). Remove cutter and filter ( a stainless steel disc with many holes in it). Tt is on a spring loaded pin. Pull the cutter blade foreward and lift the filter straight up. Clean thoroughly. I then ran a 'quick rinse' cycle with everything out, to clean the inside.
6. Reassemble - Good luck! This worked for me.

This is a 5 step process.
1 Remove bottom basket. Remove top basket (Pop off 'keepers' at the ends of the slidding shelf supports).
2. Remove bottom washing arm. Unscrew (Gently but firmly) the centre post. Lift arm assembly out.
3. Remove overhear conduit. There is one screw at the back and one at the top (Philips #2 bit). Rotate the whole assembly 45 deg right (Clockwise) and lift off of the centre post. It can be taken apart to clean.
4.Remove centre post (Pull straight up). Remove 4 screws (Star T-20 bit). Lift the whole plastic assembly out. Pop off the screen (Flat blade screwdriver around the edge) and clean.
5. Remove cap over cutter/chopper. There is a single screw at the front left (Under center assembly just removed). Remove screw (Star T-15 bit) and cap (Needs some wiggling to free it up). Remove cutter & filter. It is on a spring loaded pin. Pull the cutter blade foreward and then the filter disc up. Clean thoroughly. Run a 'quick rinse' cycle with everything out, to clean the inside.
Reassemble.

The problem is probably that the chopper/cutter filter is pluged. It needs to be cleaned
1. Remove bottom basket. Remove top basket (Pop off 'keepers' at the ends of the slidding shelf supports.
2. Remove bottom washing arm. Unscrew (Gently but firmly) the centre post. Lift arm assembly out.
3. Remove overhead conduit. One screw at back and one at the top (Philips #2 bit). Rotate the whole assembly 45 deg to right (Clockwise) and lift off of the centre post. It can then be taken apart and cleaned.
4, Remove centre post (pull straight up). Remove 4 screws (Star T-20 bit). Lift whole assembly out. Pop off the screen (Flat blade screwdriver around the edge) and clean.
5. Remove cap over cutter/chopper. There is a single screw at front left (Under assembly remover in 4. Remove screw (Star T-15 bit) and cap (Needs some wiggling to free it up). Remove cutter & filter. It is on a spring loaded pin. Pull the cutter blade foreward and lift the filter disc straight up. Clean thoroughly. I then ran a 'quick rinse' cycle with everything out.
Reassemble.

The problem is likely that the cutter/chopper filter is plugged. This reduces or eliminates water flow to the cleaning arms.
Solution:
Remove dish baskets
Remove bottom arm - Rotate the centre post until arm is free to be lifted out.
Remove conduit to top arm(s) - Remove 2 screws, one at top and one at back. Rotate whole conduit 45 deg right and lift off of centre post.
Remove bottom plastic assembly. - Remove 4 screws and lift off.
Remove cutter cap. - Remove 1 screw at front left. Lift off plastic cap. Remove cutter filter. It is on a spring loaded pin. Pull cutter blade foreword and lift filter disc straight up. Clean filter disc thoroughly
Reassemble!

Your broblem is probably the cutter/chopper filter. This is located under the bottom cleaning arm assembly.
Remove the baskets.
Remove the bottom washing arm. Unscrew the centre post and lift off the arm
Remove the conduit to the top arm(s). Remove screw at top and at back. Rotate the conduit assembly 45 deg right and lift off.
Remove 4 screws (Star T-20 bit) and lift whole assembly out. Pop off screen and clean.
Remove cap over cutter/chopper. There is one screw at left front under assembly just removed. Remove it and the plastic cap (Needs some wiggling to free it up). Remove the cutter and filter. It is on a spring loaded pin. Pull in foreward and also lift the filter disc up and out. Thoroughly clean the filter.
Reassemble

its the same water that rinses both sections just different arms, the one directly under the top basket usually cleans both cutlery and top basket. it could be a posibility that the arm itself is contaminated with waste that is too large to push through the holes and is now breaking up making a mess on teh glasses. (I have seen it before with melon seeds) remove the arm and rinse backward under a tap.
make sure that the salts are topped up as well this is very important if the salts run out you will not regenerate the resin beads which are used for water purification on the last rinse so this could also rinse with dirty water.