Buffer spring, retainer problem????? Help please.

Every time I rack back the charging handle to load a shell or fire a round the little retainer cap and spring that is located in front of the buffer tube spring pops out?
I have checked to make sure everything is lined up and it looks fine.
Anyone experience this before? what could be the problem?
The upper is lined up perfect and everything looks great when I lock the upper to the lower then one pull on the charging handle and "boing" the stupid retainer cap and spring pops out!
I see the groove on the upper and it is lined up, WTF?
T.I.A. and please excuse a slow update as I need to leave for work.
[Beer]

Sounds like the buffer tube isn't screwed in to where it should be. What kind of stock do you have on it? The buffer tube tightened into the receiver is what actually retains that plunger. What's probably happening is that it's being retained by the buffer instead, and when you charge the weapon it's pushing the buffer out of the way and releasing the plunger into the open bottom of the bolt carrier. There are other possibilities (damaged parts etc.) but the scenario above seems the most likely to me. Look at your lower with the upper off and ensure that the buffer tube protrudes over that plunger hole. If not, you found your problem. If things are sloppy enough that you can get the plunger in and out even though the tube is tightened where it should be, it's time for some new parts.

That could certainly be the problem.
I will check when I get home and report.
Thanks, John.

Problem solved!
Thanks Madrabbit!
Easy fix, the stock tube had backed off just enough where when I re- tightened by hand it did not catch the spring retainer cap.
Later, back to the range![Beer]

If you are using a collapsible stock with an unpunched retaining ring, you might want to use a drop (just a drop, don't coat the whole thread circumference) of blue loc-tite to make sure the retaining ring isn't losening up. Sometimes, the tube can be at that just barely retaining spot and allow that little nub to wiggle around or too the tube can be at that just barely too deep spot and wedge the nub down in it's hole if the buffer tube has a little room to flex.