Wclays said it.
You need to look at the Rxers that are giving you a 999. L and R that far away from A & B is not good. But, If I recall correctly, the 999 indicates something specific. You might drop Andy Kuntz a PM or call Horizon.
Frankly, The only time I have seen a frame loss is was when we were testing the fringe areas of our site, A frame loss is when all RXers did not get one data frame. I can't recall ever seeing one registered. But, there is a video that shows proper antenna positioning. Each Remote needs to have the antenna polarized differently. That way you never wind up with all Remote antennas cross-polarized to the transmitter. Carbon fiber and metal near or between the Transmitter and the Rxer's line of sight is not good.