Thanks for the reply guys.
SC Tiger, the problem occurs with multiple magazines.
CPO15, after further research the polymer frame is visibly warped inward on the left side. I was able to realign the slide on the frame enough that the mags drop out freely with the slide forward. However, they wont drop with the slide locked back (i.e. last round - empty mag). Also, it takes two hands to depress the slide-lock release.
My father-in-law got the gun used in a trade and I don't know how old it is. He has put less than a box of rounds through the gun so I think he probably traded for a bad gun. Kahr warranty specifies original owner, within 5 years of purchase, and must have purchase receipt. I guess I'll have to find a local gunsmith - warped polymer is beyond my capabilities.