I have replaced the seat two times. I can tighten it in for about one turn and then it just spins. I've replaced hot, cold and shower stems and washers but have a persistent leak (cold.) I can't feel anything wrong when I run my finger around the open pipe (minus seat.) Meaning no roughness or anything. I've used pipe tape thinking it might help. But - no. Is there a way to fix this without calling a plumber? (Like pipe compound or something...?)

THANKS, in advance.