If you're still having delivery issues in any of your private conversations try "reset secure session" from the chat menu. It worked for me with a couple of my contacts.
Does performing a reset impact messages already in the conversation? I don't want to lose chat history but I do have one convo that has missing messages between my phone and my desktop app.
Nope, existing messages are kept.
Great. Thanks!
For groups, I think you have to go to each member individually and reset it in the chat menu there.