Hey Sophie,
Here is how I fixed it. Of course, I am using the 2" MPCC so this may not apply to you.
On my 12" dob and my 8" newt, all I had to do was as described by Wookie, you unscrew the 1.25" barrel on that adaptor and the M42 thread left behind screws into the T ring of the camera.
Check the primary mirror screws aren't also screwed all the way in (meaning the primary is right on the bottom of the tube) as they do this to secure the mirror during transport. If you loosen these, you may get focus as DJ N suggested.
Cheers
Chris