On some nuts that have been slipping, the problem I have had is the glue.
There has been a lump of glue which has stopped sticking, but it gives the nut an irregular shape, so it will not sit properly under the strings.
I suggest that you loosen the strings and pull the nut out and inspect it. If it has a dollop of dead glue on it, carefully remove the glue so it is square again and it may sit under the strings without moving.
Also, this could be an opportunity to get a new nut made and invest in a set-up, with new strings, and possibly an improved tone?