When using wireless synchronization, the reader will extend the listen time if it hears a charge pulse from another reader to slow the reader down a little. This continues until the charge pulse is not heard and the reader charge pulses are in synch.
The signal strength of a charge pulse that will trigger this is adjusted using a screw on the reader. If the level is too low then any noise can cause the reader to adjust the timing. If it is too big than it will not wirelessly synchronize with other readers.
To do this, turn off all other readers and possible noise sources. Turn the screw counter-clockwise to the limit. Select "SYNCH LEVEL" on the Tuning Indicator and turn the screw clockwise until the RXSS- LED turns off.