You described that your addon code was using a command "ulx adduserid".
I believed you knew where, I'd have no idea.
If the command isnt working, you'll get an error. If your feeding it wrong info, but info matches what the ulx command thinks is steamid, uniqueid or ip, then that info still gets added, it just doesn't match when the player joins.
When at least one of the players that isn't working is connected to the server, after the addon command has run, from server console, run "ulx debuginfo". It will tell you a filepath of a txt file it creates. Attach that file to your next reply, and tell us what player.