Hi Simone
I had a issue with my 10 inch BINTEL Dob too. The only eyepiece which would not focus was the panoptic 27mm
all the others were fine. I had to pull out the 27mm eyepiece about 5mm out of the focuser and lock it into place, then it would focus. I found out my problem which was that the locking screw on the focuser closest to the tube needed to be loosened more. It made the focuser extend out and now it works fine for me.
I hope this helps you or anybody who may have the same problem.
Regards
Michael