Hello seckin onder,
are you using external antenas? If so as the distance is short you maybe saturating the transceivers, in that case try to turn down power and see if there's any difference. Also, try to set the bandwidth (CBT) to 40 MHz. Anyways, RSSI should be a bit higher for that distance, around 65 dBs.
It may also be a problem of where you have set the units/antennas, and there's not enough clearance. You need to check nothing is blocking the sight of view between the P300s/Antennas, and this is critical for things very close to the units.
Another possibility is that you are near an airport, police station or similar, so the channels could be interfered and lowering throughput. Please, select automatic channel selection so that P300s select the best available channel to use.