I'd recommend trying the following:
1. Open the VSL mixer and click the power button in the top left hand corner. Test the input in your DAW.
2. If the issue persists try opening the VSL mixer, clicking "Setup", then clicking where it says "Firmware version". This will prompt you to "Force firmware update". Click on that and allow the firmware flash to finish.
3. If the issue is still persisting try following the full uninstall/reinstall guide - http://support.presonus.com/hc/en-us/articles/210044673
If you still can not get the device to function correctly let technical support know by opening a support ticket from your my.presonus.com account.