Make sure the belt fits down into the pulley of the motor properly - if the belt is too wide it won't be able to get a good grip. Are you using a black belt or one of the amber ones with the tread? I've had those amber belts slip on me, I much prefer the black belts.
The belt should look a little saggy when it's on - you don't want a TIGHT belt. It needs to be just tight enough to catch.
Belts do come in a lot of different sizes - if you picked one up at random it might be too large. Did you have an original belt that you could use to compare size? Did it move at all with the original belt?
I have a 128 with the La Vencedora decals too, isn't it a pretty machine?
If you post some pictures of yours, someone may be able to spot your issue.
