I wonder if your mag catch is damaged or defective? My reasoning is that if the mag is sitting too low it might be able to feed from the mag until that last round when it gets hung up on the feed ramp. The fact it has the same malfunction with two different magazines tells me it is likely the pistol, not the the mags.
Sometimes the pistols get played with at the shop by employees or customers who don't know to depress the mag catch when inserting or removing a mag. The steel mag tube can wear down the plastic mag catch in short order.
You could call Kel-Tec and ask for a mag catch, then install it yourself (or with help here). It may be delaying the inevitable, but you might want to try it anyway. Let me know if you are going to ship it as I will point you to a link to save on shipping costs.