Max - The forend isn't fully seated if you don't hear a click of the latch when firmly pressing the forend in place. Try applying firm pressure to the front end of the forend rather than to the latch roller.

Ted - Are you suggesting that the following from Eightbore is shouting you down??

"Regardless of Ted's negative general comments about Model 21 rib and lug reliability, he gave good advice about going to a pro."

You must have very sensitive ears!