That happened to me after the bike sat a while.
The float got stuck!
Tap the side of the carb and try again.
Otherwise take off the floatbowl, and manually check the float (move it up and down and see if it gets stuck somewhere), or the movement might loose up some muck that's accumulated on the hinge.
You might want to check that there's no fuel inside the floats, cause if there is you've got a hole in it and need to get a new one.
I ended up having to squeeze the floats towards each other just a couple of millimeters, I think they were to close to the sides of the bowl, and got stuck occasionally.
It's possible either way with yours, might be too far apart.
Unfortunately what's also possible is that the rubber valve is worn out, and not sealing the float area properly.