Cheaper non commercial radios do not have the receiver front end filtering, as do commercial radios. So when you are transmitting to a repeater on one unit, & your 2nd unit is too close - Your 2nd unit's receiver is being de-sensitized by the transmitting unit & not receiving the repeater output.
Depending on the manufacturer, a 50 yard separation should let you receive the repeater transmission, on the 2nd unit.
This will not be noticed when talking simplex, as you are communicating radio to radio directly, regardless of close proximity.
