I have a private blockchain (using Parity) where initially, all the nodes talk to each other, but over time (usually 9-18 hours) they lose connectivity until they all drop off and have zero peers. Once in a while they find each other again here and there - one of them at least. But it drops off again just as unexpectedly.
This happens after syncing, as sync takes less than the 9 hours.
If I use reserved peers it is a little better. Even those lose connectivity sometimes but it doesn't matter because this is not going to help with this particular project.
There are a total of 30 nodes in total.
All nodes are running on VPSes. All nodes have 30303 and 30303/udp open. Most nodes are on the same network.