Valve stem keepers pops off valve stems.

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I have a 2003 Isuzu Ascender with a GM inline 6 cylinder 4.2 l engine. I recently rebuilt the engine and had a valve job done at a machine shop. After I put everything back together, I started it and after running for a while one of the valve stem keepers popped off of one the valve stems, and I replaced it. After starting it again, the same thing happened on another valve stem. Is there a reason for this happening?