Sometimes it helps to "excercise" the poppits. Take something pointy, and push them in, briskly and briefly......usually they will pop (excuse the pun) into obedience..... If they offer repeated problems....it's time to replace them.
Look on the side of your keg and figure out which model you have.... then go on line, or proceed to your LHBS and get new poppits. Remove connector stud.....(3/4" deep socket I think, or close thereto) and replace the poppit. It's not hard....just figure it out.