been there. When I replaced the heater, I eventually discovered that pex standards have changed. The new compression rings don't work on the old pex. Whoda thunk eh? After three unsuccessful attempts at joining the new tank without leaks, I consulted the local plumbing place and found out that my old pex is too thin for the new rings. Got the adapter rings, and perfect seals first crimps.
So know yer pex before you crimp. If you are dealing with pex.