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Mechanically speaking you have to able to get focus at the scope. If you imagine the DSLR is the eyepiece you have to be focussed there.
Sounds like your scope focal point is inside ( or possibly outside ) the range of the focusser\DSLR movements even with any other optical device. Knowing sods law it will probably be just inside the range your setup allows.
Either you can measure the exact stated focal length up the tube and out to the focusser from the diag to see where it should theoretically be.
Or you can do what I do when building. Take the whole focusser off and do a test with hand held camera.
It possibly sounds like you may have no alternative but to lift the primary mirror up the OTA to get the sweet spot.
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