I can only use one audio program at a time with Tascam US 16x08...why?

[KnuckleHead]

I always have multiple audio programs opened at the same time with all my interfaces, (I have five). But with the Tascam US 16x08 interface I can only use one at a time. My notation programs will state: MIDI/AUDIO device not available if I have my DAW or any other audio program opened. Why is that? Is there a work around?