Bad fuel pump, clogged fuel line, pinched line. Breather tube is ruled out since you'd be complaining about a caved in tank.
Lift the tank and measure the flow rate. You're going to have to jump the fuel pump relay. I believe it's the brown and black/white wires. Not sure if they're the same color as the newer bikes.
First measure to make sure at least 12v is getting to the fuel pump at the plug.
With the fuel line disconnected and fuel pump relay jumped, turn the ignition on for 10 seconds with the pump flowing fuel into a graduated cylinder. You should have a minimum of 167cm^3 (5.6oz).
If the voltage at the plug is below 12v, there is a problem in the wire harness. If the voltage is good but the amount of fuel that came out is below those specs, there's either a clog in the fuel line somewhere. If there is no clog, the fuel pump is bad.