1) Film misregistered. Claw stops in the middle of sprocket holes which, in turn, stops the camera. Reload film or move the film slightly and close the lever.
2) Shutter dial rotated full clockwise (locking lever control pointing up). Now locked, this stops the camera from running.
3) The camera is overwound. Bolex states in the manual that it is “impossible” to overwind a Bolex camera. Wrong. You can overwind a Bolex camera motor. There are two gears with a different number of teeth on each. One gear has what appears to be a double-tooth. This functions as a stop to prevent winding. Excessive winding torque can case one gear tooth to "jump" over another tooth. This, typically, limits the run to a few seconds before lockup. Rewinding the motor with about a turn or so results in the same lockup.
4) The shutter has an internal problem.
5) A loose piece of dirt and/or sand has found its way into the inside of the camera and is wedged as a "lock" for meshed motor gear(s).
6) Camera speed dial set to less than 12 FPS. Motor may or may not run. Dial does not have to be fully rotated counter-clockwise to stop the motor. The drag on internal gears and lubrication will affect motor stopping at below 12 FPS. Some Bolex cameras (model dependent) are marked and can shoot at 8 FPS.
7) "Release selector" set to "lock." The "Release Selector" is located just forward of the shutter control lever. The is a small chrome lever has a small protrusion just slightly outside the shutter housing case. There are four positions:
When the lever is rotated up, the camera is locked, and the motor will not run.
When the lever is centered, the camera is in "single frame" mode.
When the lever is down, the camera is in the normal, run position and will film as long as the run button is held down.
While the camera motor is running, the lever can be pushed down a small, additional amount. This will engage the continuous or “run lock” position often used for self-filming. If the run-lock / continuous position is selected, winding the motor will cause the motor to free run.