Right break light not working

I have a 2011 Honda Civic. Recently, one of the
break lights stopped working. Both tail lights go on
with the headlights, but only one of them (the left
side) gets brighter when I apply the breaks. I
beleive there is only one bulb for both the tail light
and break light, and the bulb clearly isn't burnt out
because it goes on, just doesnt get brighter. Do
need to change the bulb, or fix the wiring..? The
fuse? Anybody know.. Thanks so much!! Eagerly
awaiting some help!
Mark