1911 Plunger tube springs wear out sooner than you'd think. Recoil springs, firing pin return springs and then plunger tube springs, in that order. They don't get any oil, except if you take it out and lube it and the wire gauge isn't very big, so they just don't last forever.
If you have problems with premature slide lock on your gun that used to work just fine, replace the plunger tube spring and it will probably take care of your problem. Has your thumb safety gotten a little mushy as you work it up and down? A new plunger tube spring will make a big difference.
Pro Tip- To replace the spring, you can reuse the plunger pins if the ends of the pins are smooth and round. To move the to the new spring, pull them off of the old spring by twisting them as you pull. When you install the pins, give them a twist in the opposite direction as you push them into the spring. Before you reinstall, I use a pair of side cutters and gently close them on the middle of the spring as if I was going to cut the spring in half. I don't close hard enough to mark the spring, but it does bend the spring in the middle. Plunger tube springs are supposed to have a kink or bend in the middle to keep the assembly from flying out of the plunger tube when you remove the thumb safety.
Speaking of thumb safety - try our Widget to hold the plunger assembly in the tube, while you install the thumb safety. They are in the Tools section!