I thought you were referring to our selector
We've done lots of research on this. The mushy feel can happen to any selector, this can usually be attributed to:
Receiver's detent/detent housing (hole for the detent and spring), if it's drilled a little off, the detent isn't sitting squarely in the selector's detent holes, and isn't riding on the detent groove with good contact.
Pistol grip and detent spring. These two have to do with the spring tension, grips that seat the spring deeper have less tension exerted on the detent, and an old or shorter spring can have less tension.
Detent. The detent you have seems pretty beat up.
On cast selectors, the contact between the detent and the cast selector's detent groove is not so good, see pic below, left is Battle Arms Dev. semi auto selector, right is a typical cast selector:
Selector itself. Cast selector's detent holes are sloppy and wider than need be. A worn detent isn't seating in the holes as well as it used to, resulting in soft and less than positive clicks when the detent is engaged.
Your detent groove seems pretty worn. The groove is now probably lower than it used be. Combined with a worn detent, it would have the effect you have now.