My mod won't work multiplayer

The mod I made isn't working properly on multiplayer. When I try to connect a client to a LAN world, the world doesn't let it on even though the mod runs perfectly fine in single player. I tried looking for other people with the same issue, but they get a different error message after trying to get on the world. Whenever I try, it says "Connection timed out: No further information". Is it the mod that has an issue or the client setup?