Is the blank area at the beginning of the roll or the end?
Did you follow the loading procedure for the back, advancing the film with the knob until it detects the 1st frame.
The Rollei magazine has a sensor that detects the start of the film, it's a swinging roller at the top of the chassis that moves when the thicker film and paper advances past it. If you did everything correctly, then it's likely the film magazine may need a CLA. The mechanisms that detect and meter the film are quite precise and susceptible to old thick grease, especially for something that is 40+ years old.
Hi. Sorry for the late reply. Now my film back is having issue: cannot advance the film if it's attached to the camera, so cannot repeat the film loading process.
From memory, something like this:
After frame 12 is finished, then keep keep cranking the film advance lever till you feel it's loose. Then magazine film knob (14) will pop out. Frame counter shows 1.
Replace the film, push the winding button (16) into the magazine. Since we are on frame counter, this means we are ready to shoot, right?
OK I think I
miss this part: before pushing the winding button into magazine, turn the magazine winding knob (14) until it locks
This video illustrates that part (around 3:10)