So its not the fact that rcv refuses to send an ack,it just doesnt see the transmission.Either its not listening at the right time or the ports are not set correctly like you said.I would go for the ports not set properly,thats the easy bit to fix.If your rcver is listening at the wrong time then its a bit more tricky to debug.As a bit of radio debuging I made a simple radio reciever with an audio output so I could at least hear when stuff was being transmitted and its amazing how much it helps,unfortunatly I still cannot demodulate the data by ear:(