Hi, you hadnt mentioned how long you've had the amp, or how new it is, so I'm going to assume it's new to you. The Crate VTX200 has 3 channels, each with its own volume level. Let's check the simple things first:
1. Check to make sure you're adjusting the same volume knob volume knob to the same channel. The active channel will be lit with a green light.
2. Check the cables from the "Speaker Out" jacks on the amp (if you are using them). Are they plugged in all the way, or are they damaged?
3. Are you playing through a preset that you may have accidentally made "low", and may not be aware of or are stuck in it?
If none of these answers helps, there might be a problem with the input buffer in the circuit or possibly within the damping circuit of the speakers. I'd advise having the amp serviced to confirm an internal electrical problem also. If you don't have a manual for the amp, one can be found here: