I have an STI 2011 tactical 4 inch in 9mm.
I shoot it several times a month and it has several thousand rounds through it.
Today at my range, I had 3 failure to fire with a primer strike. They were my reloads but it was 1 with a Tula primer and 2 from another batch with CCI primers.
I plan to clean it all the way down to the firing pin.
Anything I should look for? Could I need to replace the firing pin spring?
Anything else?
Thanks

It is DEFINITELY not due to the firing pin return spring -- unless it is broken or badly kinked.

The 1911 is an inertial system.  As such, dirt in the firing pin channel can, and eventually will prevent the pistol from firing.  Get a pipe cleaner soaked with solvent down, into that channel and blow it out with compressed air (from an air compressor).  Blow it out in both directions, starting first from the breech face.

Of course, check they firing pin.  They can break.

It is unlikely that the mainspring is your problem.  Many have gone for 100 years with no attention.

Quit reloading with shitty primers.  Use Winchester Western or Federals.  Remington's are shitty too.

Try the pencil test:

Make sure the gun is empty.
If it has a magazine safety, insert an empty magazine.
Cock the hammer and point the pistol at the ceiling.
Drop a pencil down the barrel eraser end first.
Pull the trigger.
The pencil should clear the barrel by at least a couple of feet.