Have a 28CC Husqvarna 2 stroke engine. Used it once, now the attachments won't slide far enough in to connect to the drive shaft and

Here is the complete Illustrated Parts List. It has exploded views of the whole strimmer.

Have you changed the fuel tank vent.? Could possibly be a vaccum being created in the fuel tank, starving the carb of fuel.

Check the recoil spring in the starter housing, sometimes the rope rotor can jump out the hook on the spring.

Do you have compression, good spark and is fuel getting into the cylinder.?

Are they lined up correctly.? There is a video on youtube which shows the full assembly of this machine, that might help you.

May have an air leak in the fuel line, primer bubble could be split or the primer bulb base is blocked.

Slacken off the 'tube shaft' retaining clamps at eithr end of the drive tube. Push tube back in towards the engine and resit the shaft and tighten clamp screws, repeat the same with the gear head on the other end.

Check the status of the flex shaft end on the main drive shaft at the connector end, and at the powerhead end.

Doesn't sound like piston and ring problem. Sounds more like the flex shaft that connects to the engine may have an end that is worn not allowing for the extra load when you connect the attachments. Check the ends of the top flex shaft.