Multiple Thread Alert Issue

When setting up alerts to notify me of a process being down, i am running into an issue where it will not send emails if i have too many threads listed in the Source list.  Does this make any sense?

If i change nothing in the alert config except remove all but a few threads, the emails work fine.  If i then add most of the threads back in, nothing gets sent.

(This is in 5.6 on a Linux box)