http://www.roblox.com/Officer-BLOX-AI-Open-Source-place?id=19815564
test it for yourself. The AI jumps perfectly in solo but not online. The code where the AI should jump executes and the jump property is set to true, but no jump is conducted.
I've even ran another test: making player's character's that join jump constantly. Worked in solo but not online.
You could try adding a temporary...
I'm not a scripter, I'm going to get this wrong...
Velocity value up, then temporary a velocity value down.
Vector3 is it called?
Anyhoo, that's a... plausible fix.