I hate to be one to complain, especially when I'm an unpaid user, but I have been very frequently getting the error messages in the title when attempting to report spam (See attached image). I'd have to say that above 40% of what I try to report ends up in this error, and I can only assume that the spam is not getting reported. If I try to resubmit the spam, it says that the reports have already been sent. Looking at the error, I highly doubt that is the case. There is no way to attempt a resend, so the SPAMming party is likely getting away free and clear.
I'm using the 'Forward as Attachment' method of reporting, where I get clickable links back in my email and I have to manually report each one.