Have you used the Search function to find an answer? This has been asked often. Problems include:
- Hooking the OBi to the wrong port on your router. Assuming it's an OBi without a router, it should be plugged into a LAN port, not the WAN port.
- Read how to set you OBi to use full duplex, not half duplex. Posted many, many times.
If neither works, post back with specifics - model of OBi, model of router, etc.