I've been using Reaper for about 1 and a half to 2 years now and I've never had a problem with it until now. The problem started with the newest Reaper update, so I think the latest update may be to blame.
When trying to record or even when just playing what I have, there will oftentimes be silence for the first few beats of the drums. Generally only for 8th note based odd time signatures. For example, right now I'm trying to record a part in 11/8, but the 1 and 2 are entirely silent, despite there being a crashed cymbal on the 1, bass drums at straight 16th notes, and a snare on the 1 as well.
I've recorded with this song before the update and there were no problem with the drums, but after the update, BAM! The drums are silent for the beginning of many of the drum patterns. Not for all of them, though.
It's absolutely impossible to record like this, especially since one of the 11/8 parts has really oddly syncopated cymbal and bass hits, but the entire first 5 beats are silent with only sound on the last 6.
Oddly enough, the 4/4, 6/4, 7/4, 5/4, and 3/8 parts in the song all play perfectly fine with none of the gaps of silence. It's only the 11/8 parts that have the bizarre silence (for the first 1-3 beats in some parts, for the first 5 beats in other parts).
Any ideas on what to do to fix it?
Revert to the last version, and see if the problem's still there. If it isn't, report it as a bug in the Reaper forum.
I don't know much about MIDI drums in Reaper, so hopefully you asked this at the Reaper forum, too...