If the receiver is slightly out of round (which is often quite difficult to see), a mouthpiece will not fit correctly. I had that problem with my Schiller bass trombone, but since I wanted to replace the original lead pipe anyway, it was quickly solved.
If you bought the instrument new, it should be covered under warranty. If not, it's not expensive to replace the receiver.
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)