Ecotech S1 Vectra - problem with return union collar

[mtnrunner]

I’ve cemented the union coupling to the PVC pipe, but now when I try to tighten the collar hand tight, it gets pretty tight then seems like it then jumps threads and is loose. It isn’t cross threaded and I can’t see anything wrong with the threads. What could be wrong?

Thanks

 

[ksed]

Check your collar for a crack. It will be more noticeable when tightening the collar on the thread.