Silverback,
I'd recommend that you enable logging on your clients ASAP. I don't think that the CPU usage of that feature is even statistically measurable and it sure enables you to gain some peace of mind when things don't go exactly as planned.
One possibility that no one else here mentioned is that your client(s) may have "fallen back" to a D.Net server to fetch/flush. I don't know if you have nofallback set in your .ini setttings, but if you don't and the client cannot reach it's primary proxy server, then it goes looking for the regular D.Net servers. It's really a personal preference as to how to set this. I, personally, allow my clients to fallback, since in my opinion it would be worse to have my clients crunching randoms than to lose some of the stats on the team pProxy. Many of my clients have to run unattended for weeks at a time if necessary when I travel, etc. However, others may buffer extra WU's or take other measures.
My main point is that had you enabled logging, it would have been quick and easy to see exactly what server received your work.

I wouldn't worry too much though... WU's are almost never lost completely (it's never happened to me in over 1.5 years of cracking).
-Brian