Strange refractor focusser collimation problem

[jimlennie]

Since I have recently bought a 10" reflector I also purchased an Astro-Engineering laser which works really well for collimating it. I then used it to check both my refractors minus diagonals( 120 and ED80 ) for focusser alignment, they were both off centre slightly and after adjustment are now exactly right. I then placed the laser in my 1.25" diagonal and checked both scopes again and the laser spot was still central on the aperture mask that I made for each refractor. I then did the same thing for my 2" mirror diagonal and all was ok with that as well. Finally, I did the test again with my more expensive 2" dialectric Burgess diagonal and the laser spot was about an inch of centre on both scopes. This is puzzling and I still can't work it out, I can't adjust the mirror in the diagonal as there are no screws visible. The star test is ok though which I find strange considering the Burgess diagonal, according to the laser, is off centre. Has anyone here come across this before ?

Thanks

Jim