I've got an odd issue that I've had for a while. Part of it I'm pretty sure we can blame on unsupported OS but more recently I've run into the same issue on a supported OS. I've got a collection of old servers (mostly VM's but a couple of actual boxes) that are 2003 R2 or 2008 R2 that always show as offline in Kaseya. I can RDP into them with no issue (usually) and as they're mostly file shares people have no issues connecting to them. Here's where it' gets wonky, I have the same issue with about 6 2012 R2 servers and yesterday I found out that one of our new Azure VMs running 2016 was offline and wasn't being monitored so we missed a low disk space alert.
I don't have anything that is common across the board for all of the offline units. I did run into a couple of servers where there were too many people logged in so I had to kick them off the server for me to connect but that's not true for every instance.