How about the props? Are you sure you got the correct ones (CW or CCW) in the correct positions?
If you draw some lines do you get this?
The green being the FCB. It can be shifted a bit up or down (blue line) but it needs to centered side to side.
How about a top view of your quad.
If you are only using stabilization and no auto-level (The I value will start to get a tiny bit off over time like this) you can put the FCB anywhere on the frame as the angle is the same, I tried mounting it on an arm to test this and was surprised to see it worked great in 'rate' mode (just stabilization no auto-level). Most FCBs also support offset if you ever had a reason to mount it rotated a bit (like on some + frames that you might want to be an X).