what cause the primer bulb not to pop back out when i press it in?

(Primer return passages must ‘be’ clogged) .... With a little more information - can be more helpful. What kind of (brand name) trimmer do you have?

it is a murray trimmer the primer bulb look ok i replace a new carberator on the trimmer August of 2017.

Remove primer bulb and probe passages - primer bulb replacement is a good idea during yearly tune-up > @ start of trimming season. Empty out old gas from gas tank and add NEW fresh gas .. New spark plug - new air filter - new fuel filter and adjust carburetor. Make sure gas tank is absolutely *clean* inside! May need to visit small engine - lawn and garden - repair shop. Good luck!