Could be the keyless works or, if a Rolex model, a worn crown. The inner, hex-shaped section that holds the stem fits into complementary hex-shaped hole in the outer spring/plunger assembly. Over time/with use, the hex edges tend to become rounded, which allows the 2 sections of the spring/plunger assembly to 'jump' as the watch is wound, which feels like something is slipping during winding. A common problem on vintage gen crowns. Only fix is to replace the crown or plunger assembly with a donor from another crown (read this).