There are a number of causes this could be related to.
Please ensure you have allowed the program through your windows firewall and antivirus firewall.
Some routers block incoming connections; if you know how to configure your router, try disabling the firewall or allowing incoming connections.
Otherwise, if you have a mobile network you can switch to, try that.
If you are using a VPN, disable it. Conversely, if you are not using a VPN, a VPN may help.
My copilot's aircraft is below or above me or in the ground
This is commonly due to mismatching sceneries. Please ensure you have the same add-on airports installed and that you're not at an airport that is handcrafted in one edition of the sim, but not in the other (Premium Deluxe vs Deluxe).
My copilot's aircraft is in front or behind me
Disable multiplayer and make sure you are not in the same group. If you're using an online network such as VATSIM or IVAO, check out VATSIM/IVAO for more information.
Unfortunately, weather cannot currently be synced between flight simulators. If the sea pressure, ISA, temperature, and other weather-related aspects are different, you will experience desync.
Aircraft uncontrollable after transferring controls
This is due to the autopilot constantly setting the trim for the copilot to the point where the aircraft will nose all the way down.
The current workaround for this is to disable autopilot before transferring controls.