Sewer line cap won't stay in

I have a cast iron sewer line and we recently cleaned it out due to tree roots. We put the same cap that we have always had back in, but we cannot get it to stay tight in the pipe. When pressure builds up, it pops right out. I tried to buy a 3 inch and it was too small. A 4 inch was too large. I can't seem to get anything to work. The old one just turns and won't tighten. I don't know what to do to my old one to make it have a suction. Is there something I can clean it with or clean the pipe with? I need help fast.

Check if there are still threads left in the trap adapter. You may be able to clean them with a wire brush... If the threads are shot use a lead Fit-All...

As for the pressure build up get the line cleaned by a pro! It's still clogged!