several times the 2 blade fan loosens on the water pump

[Oberfranke]

I am at a loss. Maybe someone can help? I had a new water pump, leakless style, installed by a repair shop. It was mounted the old 2-blade fan. After about 100 miles, the fan was loose and the mount for the fan shaft key in the fan was defective. I then installed a new 2-blade fan and reinstalled the water pump itself with great care. After again about 100 miles the same problem. The cotter pin is still present in its position, but the fan again loose. What went wrong? Maybe the cone on the shaft does not fit? I am pretty sure that the fan shaft key did not slip during installation.