Some times the string get wrapped and knots up, other time it may be the string your using if too big. if it the bump type to advance string and it advances when you rev it up or lightly bump it on the ground the spring may need to be sprung more to put more pressure on the release gaget.

Most, if not all, of the trimmers in use today, have the "bump feed" type of string advance, where you rev the engine up and bounce the trimmer head on the ground several times to advance the length of the string!
If the string doesn't advance, there is a problem, caused either by faulty string winding by the owner, or sometimes just caused by a temperamental machine! They do have a mind of their own-ya know? You have to talk nice to them, or you're in for a lot of trouble from it.
JMHO: Rusty J.
Note: To save yer sanity--go to the mower shop and buy the trimmer head that uses the heavy orange or red, plastic lengths of cord, which will cut off an elephants feet without breaking! BT/DT: Rustyj

Damned good advice from Rusty
The string head should just pull apart.
Be sure to wrap the 0.80 red or orange in the correct direction and very neatly..

What is the correct way to wind the string? I never learned. And sure would like to. TIA