Cant remove trimmer head from homelite st155 trimmer

SOURCE: Replacing the bump head on homelite string trimmer

undo the 2 screws on top of the carb. The bulb is part of a 2 piece cap undo that and it fall out.

SOURCE: cant get the spool off homelite trimmer

Unscrew the bump-knob and the rest pulls off. If it won't unscrew you could always cut it off and buy a new one from ALM Manufacturing 01423 797208

SOURCE: Homelite strin trimmer head wont rotate

The only way to access the bottom of the metal shaft is to break it. This is fused on at the factory. It sounds like your flexshaft has twisted in two. Remove the flex shaft from the engine end and see if it is in one whole piece or is damaged in someway. If it rotates at the top but doesn't rotate at the string head this is probably the problem. However, Homelite no longer offers just the flex shaft for purchase. You would probably have to purchase the shaft and the tube as a unit.
But if you are saying the spool and string is not turning at the bottom it could be that the cup which holds the string has worn and can not longer be turned by the shaft. Replace the cup that holds the string.
Another possibility is that the shaft have moved forward, away from the engine and just need to be jammed back in, and retightened (the handle grip and the boom clamp).

SOURCE: removing spool from homelite 13 inch electric trimmer

The entire cap does not come off. You push the buttons in on each side and only the very top of the cap comes off.

SOURCE: Have a Homelite gas trimmer. Cannot remove the

Not sure but have you checked to see if it might be left handed thread