Finding the right tensioner is important and sometimes difficult.
Some keyways have a slight curve at the top which makes it hard to tension in one direction. The tools doesn't lock properly and want so slide out. If the lock can be turned in both directions, try the other way around. (Often the case with door lock cylinders; but not padlocks for instance.)
If that's not possible, try different thicknesses and sizes if you have them. Sometime it's not the perfect fit that works, and you have to do with whatever works.
If you're not already, mount the lock in a small table-top vice. This might get you better control than when holding the lock in the hand.
Give up and use bottom of the keyway instead.
(If at all possible of course.)
