Taurus 1911B, 2007 build date says Taurus' website.

Problem: When the slide is locked back and I sling shot the slide with a full magazine the slide stop drops approximately half way, preventing the slide from picking up the new round and going into battery.

If I thumb the slide stop down the slide goes forward and the gun goes into battery.

I tried both of my Taurus magazines, both resulted in the same problem. It worked with both magazines all previous & last time I was at the range, the only difference is that it was a different day, and I cleaned the gun in the interim. Maybe this is my fault??

I also dried pulling the slide back with no magazine in it, and there was no positive force pulling the slide stop down and out of the way, I have to thumb it down.

What should I look for to figure this out?