FTF or FTRTB on a G-27 Gen 4

I just bought a Glock 27 Gen 4 and have put about 125 rounds down range. In the last 50 rounds I have had a few failure to return to battery. They started in the last 3 mags shot. A few days ago, I shot about 25 rounds with 1 FTRTB or FTF in the 2nd to last mag.
When this has occurred it seems to be about 3/4 ways out of battery. All I need to do to get back in action is to pull back on the slide and release and all is fine. It has occurred with Federal Champion (a few times), Federal HST (1 time) and Tula Ammo (1 time in that 25 round session).
One observation I have is my 3 mags have strong springs and the 9th round is a bear to get in. This has not occurred after the first round fired from a mag.
Thanks in advance for any responses and I hope I have given enough infomation.