Welcome to the forums!
You gotten some good advice above.
FWIW, I'd like to add my 2cents. When you say "..first 6 mags go fine...", I'm assuming that this means six loading of the magazine that came with the gun vice six different magazines. If this is correct and considering that the single magazine worked fine for multiple loadings, I wonder if the front skirt of the magazine follower has broken off. Here's a photo that was posted by moosetang that shows a broken follower and the piece that broke off:
If you find this to be the case, we can look at it further.
Regards,
Greg