Hi,
ok, first thing. MMO is ( I think ) not really recommanded with Photon Cloud and Photon Unity Networking since it's a "room" based abstraction of multi user. So you will find that over time, switching to a custom based photon networking solution will likely be the way to go if your MMO really gets into high CCU since the whole streaming and multi user networking technic is completly different than a room based system. You should get in touch with Photon support to clarify all this.
Now, with your actual issue at hand, what does it says in the Unity console? the error that Photon or the bridge with fire is important for me to point you in the right direction.
bye,
JEan