Magazine drop-out is a fairly common problem with the PT-1xx Taurus pistols. IF not caused by inadvertent contact with the mag release button (and THAT is the MOST common cause), it is usually one (or more) of three possibilities: either the mag catch is cruded up and not engaging with the slot in the mags completely......or, the mag(s) are poorly made (the slot for the mag catch is off location just a bit and the catch doesn't engage completely.....or, the mag catch is defective. Defective mag catches are not uncommon on these (the part is plastic, I believe).
The best course of action would be to address these three possibilities in order.
So, first clean the mag catch area thoroughly. Carb cleaner, sprayed through the little straw that comes with the can, will tend to flush out any crud. You can reach inside the mag well with a toothbrush (or similar brush) and scrub a bit as well, after spraying, to dislodge any gunk.
Next, check your mags for fit. Snap in a mag, then wiggle it somewhat violently. If it pops loose (without pressing the release), then either the mag is bad, or the catch. Check to be sure that the mag goes all of the way up into the well....and that you get a solid "click" when the mag is seated all of the way. If the mag(s) are not solidly clicking in, then it COULD be that the catch slot in them is off position a bit, or burrs could be causing the catch not to fully engage. If you detect any burrs on the edges of the slot in the mag, these can be removed with a small file. If the slots are off just a bit, that can be addressed with a file as well, though I wouldn't try it unless you understand what you are doing. Also, check to see that the mag base plates are not interfering with the mag going all the way in - if that is the problem, then a bit of sandpaper will fix that, easily.
Lastly, the mag catch could be defective or broken. Shine a light down into the mag well from the top. The mag catch (the important bit) will be on the right side (opposite the release button) and should look complete (have a straight edge) and not worn. You can test your mags again, by inserting a mag with the follower and spring removed but base plate ON, then checking with a light from the top of the mag well (through the top of the mag body)....to see if the mag catch seems to engage fully. The mag catch can be removed from the gun for inspection, as well. I've never done it, so I can't advise you on how, though.
The last step, if you don't feel competent to address these possibilities, or if you can't find the problem and correct it - is to send the gun back to Taurus for repair.