It's possibly the setting of "Run only when idle", which means you have to NOT be on the PC when it's running.
In advanced view, go to options, computing prefs, and click on computing prefs
Then in the section section "When to Use", uncheck the second option "suspend when computer is in use"
That was what I did to solve this problem when I first downloaded BOINC (not long ago)
waiting for a TdP prime...
Proth "SoB": 44243*2^440969+1