I hope I am in the right place!

Uptodate FLYMO lawn edger that will cut for a few seconds and then the
plastic thread disappears inside the spool.

I have tried everything in the way of re- threading and it will never
cut for more than in few seconds.

I am not asking it to do anything extraordinary, just mostly straight
edges, no rockery etc.

I have tried several Flymo edgers over the years and always have the
same problems. I can't see I am doing anything wrong.