Have you thoroughly and repeatedly cleaned the receiver with a brush and something like 409? I mention this because it took be about 4 such cleaning to get all the built-up crud out of my Buescher 1924 tuba receiver -- at which point mouthpieces were seating about 3/4" further into it.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)