Around 20 to 30% of the Buren cal 11 through 15 movements that I service have a loose cannon pinion. The cannon pinion in a watch is a bit like a clutch in a car. When you set your hands, it slips so that the hands can be set to whatever time it is. But if it is too loose, the hands will slip and the watch will not keep time even though the movement is running properly and keeping time. This part in these movements is off center and the movement has to be disassembled to remove it. The first pic show the cannon pinion and it size in relation to a penny.

The next pic shows the cannon pinion in the cannon pinion tightener. This tightener had to be modified just for this part. It is tightened by punching a small dimple in the side. This is tricky - too small a dimple and it slips too much and too large a dimple and it will not slip easily enough. It can take several tries to get it right. Even some of the NOS ones that I have found are too tight and have to be adjusted.

The next pic shows the cannon pinion installed on the gear.

The next pic show the gear back into the movement.

The last pic show its location with the bridge reinstalled.

Most of the time this happens because the watch has not been service on a regular basis.
So, if you hands a slipping (loosing a lot of time), this is the most likely cause.