Although the advice above is spot-on concerning over-tightening - which might cause the fan "stages" inside the motor to foul.... I suspect you mean the cooling vanes which are on the back of the motor? This is exposed when you take off the black plastic casing
The way these are designed is very crude - they are simply forced on to the motor spindle, relying on friction to keep them in place. I have had motors where this has come loose so it no longer turns true. You need to use something like a small bit of metal tube and CAREFULLY hammer it back on to the spindle. 2 problems, firstly it is a b***rd to get it true again, and secondly if you force it on too far it will foul the casing when it is re-fitted. But with a little bit of jigging around you can sort it, unless you've forced the hole open too far.