I'm coming in a little late on this topic...however...I have had the same problem. I bought a taurus millenium pro pt-140. The first time I took it to the range the mag would drop, released by the recoil of the weapon. This especially happened when I would double tap, because I was not getting a good hold on the handle. being a polimer body, it does not have grip screws to tighten, so that is not an option, the thumb is not hitting the mag release. The release is just very shallow.
So...I have solved this problem.
Here is what you will need...1 roll of black gaffer's tape (like they use for video, photo, stage productions). It's a lot like duct tape but has a cloth back instead of the plastic that duct tape has. It also will not leave a gummy sthicky residue like other tapes. You can pick this up at a local stage shop, some camera stores, or bhphoto.com online. It will run about $13.
Cut a small strip about as wide as the clip and about 3/4 as long as the clip and place it on the left side of the clip (if the gun is in shooting position, pointed away from you). This will solve 2 problems - 1. It will shim the magazine a bit closer, causing the mag retainer to go deeper, and add a bit of friction to the mag so the recoil does not cause as much tension. The type of tape is important - black - so it doesn't look stupid, and gaffer - for the friction, and the lack of gummy residue inside your pistol.
later guys! hope this helps. Sure...I guess I could have sent it back, or got it gunsmithed, but I think this will be an ongoing issue, and so far this seems to have done the trick. see ya at the range! If anyone is interested in seeing photos of what I'm talking about, let me know, and I'll see what I can do.
-t.c.