I'd suggest contacting Tech Support. Each link needs to be evaluated individually.
If you have good line of sight, do you have the same Distance set between each pair of RB/NRBs? This affects link timing ACKs.
Have you run the Channel Optimizer from your RBs, to see if the bridges consider the current channel to provide best bi-directional throughput?
If you have tuned each link for best performance, then look deeper into the pair of 7731 bridges Support Info files, and are there periods of high interference that affect any particular links?
When tuned and not affected by other equipment, the 7731 PtP links should act like an invisible wire in the air.