My experience with .40's is that the full mags are a bit tighter.
You could insert mag with the slide back, or...
Give the bottom of the mag a good rap with the heel of your hand to get it seated or...
Download the mag by 1 or 2 rounds.
I had one new mag in my 27 that was so tight when full that I couldn't pull the slide back. Disassembling the mag and trimming the extra plastic so the spring fit better, loosened it up some.
Luke 22:36 He said to them, ď... if you donít have a Glock, sell your cloak and buy one."
°Glock don't need no stinkin mods!