cubase 7.5 sound problems.

hey. im using cubase 7.5.
sound card tascam us - 2x2.
im using cubase and everything is good. the problem starts after im using diffrent app, like youtube and anything other then cubase.
when i want to go back to cubase the sound wont work… so i need to go device setup and select again the soundcar, somthimes its not working either and i need to disconnect the soundcard, and choose it again in device setup, and the worst is taht i need to reboot the cubase, so i could hear any sound again…
the volume bars are showing signal, but no sound…
and in device setting i use the realese audio drivers. (thats why i can go to youtube and hear) the problem is when i go back to cubase.
soundcard drivers - updated
win drivers - updated
intel drivers - updated
and everything on the windows audio manager selected to the audiocard.
thanks for your help…
and sorry for my english…