I believe the Pex being so stiff, and wanting to stay curled was putting so much side load on the compression nut, it just felt as if it was getting tight against the sealing ring. At second try, I held the tubing so it was straight into the valve with no side load while tightening nut. All is well so far no leaks. But I'll confirm there were no inserts in the parts kit, nor did the instructions show any for the optional shortening of the lines. 9 days and Delta support still has no answer to my question. If I had it to do again, the unsightly under sink clutter looping of the very long amount of tubing would still be a consideration for me. I would think Delta had done some research on the two different methods before offering the option. I went to local hardware with piece of cut off Pex and tried to force a brass insert into it, it could not be done with just my hand and arm pressure against a hard surface as with the soft poly tubing. Are there special diameters of insert for 3/8 Pex??? If so my hardware store did not know of them.