Is there a red box with the message "*** can't find a route" above the bus ?
Is the road before the bus free so it can move ?
Have you set a minimum charge at this stop ? In such case, your bus might be waiting to be full, check in the bus'window if there is a yellow part on the loading bar.
I guess you have properly set a non-null power and a non-null speed for you bus, else it would not even have left depot.
A screenshot of in game vehicle info dialog would be helpful, as well as dat file. Wild guess: wehicle is underpowered and too heavy when fully loaded.
Sounds weird but possible too, what is the total mass and the power of your bus ?