Whats happening here?

The above diagram (not to scale) shows a simple radio system, consisting of a repeater located on top of a building (X) and several portable radio users (A; B and C). User C is about 1km away from the repeater and users A and B are standing next to each other, about 15km away from the repeater. The system is on UHF and propagation seems okay since this is a suburban environment with some obstructions.

The repeater is running 25W; is using a single folded dipole and notch-type duplexer. The ERP is around 10W and the repeater sensitivity measured at the duplexer ANT port is around -116dBm. The building is 10 floors high and the antenna is mounted on top of the elevator headgear room.
All the radios used on this system are the same model; use the same antenna and are programmed the same.

Whenever user A transmits, she cannot be heard by user B. Similarly, when user B transmits, she cannot be heard by user A. However, when user C transmits, she can be heard by both user A and B.

What is wrong here?
If you need more information, please ask questions in the comments.
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