Diagram of top load bobbin case in a singer heavy duty 4423

Be sure the bobbin winding spindle is full to the left.

You may need to clean/oil the machine.

http://www.menwhosew.com/how-to-clean-and-oil-the-singer-heavy-duty-sewing-machines/

https://www.sewingpartsonline.com/singer-parts-4423.aspx?sort.ss_cat_sort=desc&index=1

Did you loosen the handwheel to wind the bobbin? If so, it needs to be tightened before the bobbin winder is released.

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Customer Support ,,,,

If you've cleaned and oiled it, change the needle to see if it is catching in the wrong place and re-thread, of course. If none of that works, it's probably the timing ... find a good repair guy.

it would be a good idea to have the sewing machine checked to see why the bobbin case is loose.
here is a link for parts for the sewing machine
http://www.sewingpartsonline.com/singer-parts-4423.aspx

Did you change the needle after you got the timing fixed? Are you sure the timing is correct?

First make sure the needle is the correct size and fully inserted into the machine. Test

Then if that doesn't solve the problem check the timing.

Good luck

Put a felt spool pin holder (an extra one comes with machine and it's either red or white felt with a circle cut out of it in the center)...remove bobbin, set felt circle in bobbin case to keep bobbin from wiggling. The back and forth motion is knocking the thread out causing it to lay flat against top of the bobbin when you start machine, but felt puts bobbin in correct position! Make sure thread falls into tension slots and carefully pull thread back to left of silver piece (stitch finger). Pull up your bobbin thread. Put in new needle since "bird nesting" or jam may have bent the one previously used. Now it should purr like a kitten...smooth sailing! This is how I fixed my Singer Heavy Duty 4423 when I realized the problem is that the bobbin case is a bit wide and felt takes up slack, raising bobbin also to keep bobbin thread moving smoothly through so it doesn't jump out of tension and rest on top of bobbin!!! Works now!
Jimmy

Make sure thread is not laying across the bobbin when you look down into the plastic slider that covers the top loading bobbin case. The bobbin on mine I observed was loose in the case so that as the machine ran, eventually the bobbin thread would jump out of the stitch finger (the silver piece to the far left of the bobbin case, past the tension slots). I solved this problem and so can you! Take the extra felt spool pin felt in your accessories kit that came with the machine, remove bobbin from case and drop this small felt circle into the bobbin case. Replace bobbin. On the 4423, the bobbin thread goes counter-clockwise and drops into the tension discs, and goes all the back until it falls into the space left of the silver stitch finger. Make sure take up level raised all the way up! Then pull the needle thread toward you until bobbin thread comes all the way up. If you start sewing and you actually can SEE the bobbin thread laying across the bobbin, STOP and tethered because that would mean your thread did not slide all the back behind the silver stitch finger--however, I assure you, if you go one step at a time, you did it correctly! You will be smiling in no time! Why? The felt helps to make the bobbin rest in the correct position to keep it from jumping out of the groove. I hope this helps you.
Jimmy