DELETE

[red_star_republic]

DELETE

[858casper858]

What kind of rifle?

[FeuerFrei]

Usually the detent spring is weak or the hole is too deep in the grip.
Or...
The detent has no point or too little. Could be the safety detent hole is out of spec.
Somehow.
All of these parts are cheap.

[Noobert]

Take some pictures of the detent and the dimple in selector switch

[willm952]

I had a similar problem. If your lower parts kit is fairly new, try this.
Dap some Remington oil or clp on the selector while its in fire position. Then switch it from Fire to Safe to see if you notice any improvement. An Army vet showed that to me. Selector worked like a champ after. Clear your weapon first of course. CYA for me.

[rareair]

Remove the pistol grip and re-insert the detent and spring making sure all parts are sitting properly.

[HotRails]

still need to know what kind of rifle.. AK/AR/?

[red_star_republic]

AR15

[goober]

some selectors have munged-up detent divots... if the detent & spring are installed correctly and the hole in the lower is to spec, check the selector. it may have crud in the divot, or none at all, or otherwise be the culprit.
(ETA this is what Noobert was suggesting).