Why doesn't my Vox tone lab st work well with my line 6 spider II 75?

so i recently bought the Vox tone lab st, because i saw awesome demonstrates of it, and it was recommended to me by few people , but when i use it with my amp, the distortion sounds pretty crappy, the clean is ok though. is stacking both modeling technology not good or something? if someone can direct me that would be cool, thanks.