Hey mdtnman - welcome to the site!
I know you said you tried 4 mags.. were they all used?
Are you able to ask the previous owner if they had issues?
Do the mags have mag extenders on them?
Does it seem to not chamber the 2nd round every time?
Is it only the 2nd round when it's full, or does it happen on the 2nd again if you only put 5 in to begin with?
Is this your first glock? did you come from other manufacturer type guns?
Meanwhile, I would try a fresh mag from your local shop and report back... maybe the mags are old and could use new springs...