Is this something that happens every time? Or just sometimes? Do you see the .DEBUGINFO file in the deployment folder in the server?
The VAD takes note of the current build number and if there were changes to the project since the last build. If you make any change to the project, the VAD will not let you debug, because we can't be sure that the designer will have the same flow path than the deployed project. So, when you make any change to the project, you need to rebuild and redeploy in order to be able to debug. In this case the VAD will disable the start debug button.
But if you build -> deploy -> modify the project -> build again (and don't deploy), the VAD will enable the debug button, but will look for DEBUGINFO files in another folder, so it will not find them and will show you the list empty. Maybe this is what you're experiencing?
Hope the information helps and let me know if this is not your scenario and need additional insight.
Thanks.