The OP mentioned a T9; I recently replaced the slide stop spring on my T40 and I believe they're located on the left side of the frame on all T models.
It's a small-diameter, triangular spring with one end bent to index it in a hole in the frame and is held in place by a Torx screw. The front end of the spring sits in a circumferential groove cut into the left side of the slide stop. When reinserting the slide stop, you have to be very careful that the rear end of the spring rides on the top part of the slide stop.
The slide stop spring on my K9 is located on the right side of the frame. It is considerably thicker and rides in a groove cut on the upper right end of the slide stop. I much prefer this design.
In my case, I ended up replacing the slide stop, slide stop spring and magazine followers.
The metal inserts in the front left corner of the magazine followers were worn to the point that they barely engaged the slide stop. Also, the slide stop spring was not putting sufficient tension on the slide stop to keep it from moving laterally slightly, allowing the slide stop to drop down. Finally, the slide stop was worn to where it did not reliably engage even with the new magazine followers and slide stop spring.
Replacing those three parts corrected the malfunction and now it's back to 100% reliability.