Sealing threaded PVC pipe
We use liquid sealant all the time with great success at 1-1/2" diameter and less.


Make sure you are coating the threads completely and evenly.
Some dopes require coating both male and female ends over certain sizes.
Maybe try a different brand.
Might just use teflon tape instead.
Plumber we know for a large local company uses teflon (to seal) and
pipe dope (to lubricate) at the same time although we do not practice
the same.
If these don't work, look at your threads and make sure they do not
have a scratch or nick in them.