Question about Neonate Bc-6500d

Hello, I have two babies and the parent station. Now when I talk to one child it works but when I switch to the other one the talk function does not work every time I press the button although both are paired. We have already sent it back once and had the same problem with the new one. The connection (flash in the display) does not come every time either, so no connection can be made. But should it be possible to pair up to three baby stations?