A number of reasons can be responsible for this:

1. The hairs have stretched to the point where the frog nut reaches the limits of the slot and a further tightening could result in the stripping of the relatively soft brass tension screw. You can determine whether the hair length in fine by completely unscrewing the tension screw, removing the frog and pulling the hairs to their complete length. Now it can be seen if there is sufficient room in the slot for an additional tightening.
2. If the tension screw keeps loosening so that it is not possible to tighten the bow properly, the bow manufacturer did improper work. The hole in the stick and the screw must have the same length. Otherwise the screw cannot fasten as it doesnt touch the bottom of the hole. A violinmaker can choke the hole so that the screw can no longer loosen.

To get a better look at the mechanics, here is a drawing of the frog's cross section: