I agree with what Dom says on this.
Is your loop size correct? It is important to maintain proper loop size: 54 perforations for both the 400 ft and the 200ft magazines.
What size mag were you using, 200 ft or 400 ft? If it's the 400 ft size, which mag do you have, the 35-II B mag with the belt drive on the mag, or the later 35-II C / 35-3 (called the 480ft Color magazine) version with the gear drive?
If you have a 400 ft 35-II B mag, the cloth belt acts as the clutch, it is easily adjustable, but I doubt it would have enough torque to pull the film and strip the perfs. If the tension is too tight, you would hear a 'clicking' or buzzing sound from the film when the camera is running. Usually the cloth belts got loose, and wouldn't take up tightly. At least that was my experience with the mag.
35-II B mag, 54 perf loop
take-up cover removed
The yellow arrow points to the adjustment spring thats puts tension on the take-up belt. You can use either of the 2 posts to increase or decrease tension. Read pages 42 through 51 of the repair manual I sent you! A tension gauge is recommended to adjust properly, page 48 of the manual.
If you need a new belt, I don't know where you would find one. Arri stopped supplying those decades ago. It would be easier to get a 35-II C mag with the gear drive than to find a belt. (UNLESS, Dom has some in his parts drawer) (I do have some new/old stock 200 ft belts). The thing is, with the gear drive magazines, you then have to deal with the felt clutches... another story.