Usually the clips or retaining rings pop out because you don't have the proper preload.With proper preload the plunger should never contact the retainer.If it plunger contacts the retainer it will pop it out.When setting preload on a hydrualic lifter turn the rocker adjuster to zero lash then 1/4 to 1/2 turn to set the plunger away from the retainer.Make sure the lifter is on the bottom of the cam lobe.Do not reuse the retainer.