Go to the modular tab and select an already working instrument and remove both of the cables going from the pad's velocity output into the AM input of envelope1.
Instead link the pad's velocity output to the AM input of the sampler module using a cable. Now use the pad with different velocities. The result is that the sound always plays with the same volume. Why is this the case? It has nothing to do with the Env1 generator as the output from the sampler module should already have diferent volumes based on the velocity you used to hit the pad.
Any ideas?