Windows co-op constant disconnecting

[Faye the fairy]

My friend and i have been trying to play online multiplayer, originally i hosted but they kept getting kicked out saying the server disconnected whilst things were fine for me , we then swapped and they hosted but i then got kicked out for the same reason, we cant always get back in even with an invite code.

1. playing on windows,
2. version 1.6
3. tried compatibility mode

but nothing works, please help