I have a gas featherlite trimmer and when I press the primer bulb it stays down.

How old is it?

About one year

Hi,
A check valve within the primer is stuck. There arre a pair of them facing in opposite directions. Not a serviceable part in itself, replaced as a unit being careful to put the new one in in the same orientation as the old one, generally one tube the lines attach to is longer than the other.
Peace,
Clarke