Thanks, Jan - OK, here's how I can reproduce the problem:
Running N1MM "Classic" (the old version, last one issued) in Enter Sends Message Mode.
Fkeys are set up in the typical contest fashon: F1 is CQ, F2 is Exchange, F3 is TU
Call CQ by hitting F1. Enter a call sign hit Enter - exchange in F2 is sent.
Enter the other stations exchange, hit Enter - TU K3TN is sent (or whatever you have in F3)
Repeat, now either just by entering call signs as if someone came back to your TU, or hitting Enter again to send a CQ.
After a few "QSOs" when you hit Enter, nothing gets sent. If you then tap a dit on the paddle, the dit gets sent *and* then the Fkey message that should have gone out when you hit Enter. If, as often happens, I hit Enter twice or tried another Fkey to see if it would work before hitting the paddle, all of the unsent messages are sent once you hit the paddle.
I did not see this behavior in the WPX RTTY contest last week, using same version of N1MM Classic and the new firmware. But that was RTTY, and had COM3 set up for FSK. I switched it back to Winkey mode - I don't think I changed anything else - or is there something I may have done going CW to FSK and back over COM3? I didn't see anything.