I tried running dex on a Mongo shard log I've got and noticed that it missed a bunch of findAndModify lines. It turns out that the log has "findandmodify" instead of "findAndModify" for the command name. I hacked my copy of parsers.py locally to account for that one special case, but I'm not sure if something more comprehensive should be done. I can open a PR with my changes if it helps, but I'm not sure they're exactly what they should be (in particular, my installed version doesn't have SUPPORTED_COMMANDS in analyzer.py).
A great find. I'll take a look. Preliminary, I don't see a problem with supporting case-insensitive command names, so please do submit your tweak as a pull request, and I will incorporate it differently than as-written if it becomes necessary.