Binatone 150 's Not Working Properly.

Hi, these were a Christmas present for my daughter. After only a few uses they no longer seem to work. Both units are charged and turned on one unit is on channel 4 with (L) displayed the other unit seems to default to channel 5. When I call one unit from the other the call is not received up by the other unit . When I scan the channel on one of the units hoping it will find the other this won't work and likewise if I manually try and change the channel on one to the same as the other - ie changing the one on channel 4 L to channel 5 which the other is on it just reverts back to channel 4. This is the same when tried on the other handset in that it falls back to channel 5 and doesn't seem to detect/recognise that it should be on channel 4 like the other hand set.They had been working when used previously but they have only really been used for an hour or so since they were opened on Christmas Day. Any help/advice would be gratefully appreciated