You should not be able to slip it by hand. When properly tightened, you should be able to flex the belt about 1". If you get the belt too loose it will slip and glaze the belt surface, leading to more slipping, even when adjusted properly. If you get it too tight, you will actually deflect the shafts of both the motor and the drive pulley. When too tight and the shafts deflected, the inner and outer races of the bearings will be misaligned, leading to premature bearing damage. The damage mode is known as Geometric Stress Concentration spalling from an elliptical roller path.
I've attached photos to show you what the bearing ends up looking like if you get it too tight. The elliptical roller path damage appears first, leading to spalling (like a pot hole in the road) that grows from the edge of the roller path towards the center of the bearing race. The schematic drawing shows how the shaft can be easily deflected out of line to the motor housing.
If your belt only has about 1" of deflection when you push it, but still slips, you need a new belt. I hope that this helps.
