BOINCstats.com account manager system is only very limited supported by WCG, in fact, most of the interactions are directly between your client [version?] and BOINCStats AMS. It's very well possible that your AMS decided to connect as a different member [if you have more accounts], but if tasks are completed and no longer listed as "Ready to Report" in the client, then WCG did receive them. That is, if the AMS settings are not so confused that it is detaching from WCG and attaching again... then your work is lost. The Result Status page would tell what the state is of the tasks... In Progress, Detached, Pending Validation etc.

Where the fault lies... not going to finger-point since I noted that WCG is only very partially AMS interactive enabled [Many changes over at BS wont set into the profiles of WCG]. I know GR AMS [Grid Republic] violated network filtering and still pulled and displayed information I did not want them to have. Account closed. The only reason I have an account at BS is to see the detail and roll-up statistics [hosts hidden from private eyes ;o]

If your friend cant tell you how to config BS properly, maybe refer to the BS forums. BS has a bad habit of detaching and reattaching WCG, depending on the sequence of steps you took of attaching to WCG, then adding the BS-AMS. Somewhere long ago I figured a box needed to be ticked over there to prevent this happening [default attach WCG] and did post about it on our forums, but that's years ago.

"some of my hosts are sending completed tasks"

If your hosts return WCG results, then have you checked where they're coming from i.e. do these tasks appear on "Your" My Grid > Result Status? Compare what you have on clients with what WCG lists as having been assigned.