One problem you might be experiencing is the rechargeables are sometimes very slightly shorter than the alkaline type, not making a good circuit.
Basically the positive contact, others on a Crawler forum have sussed it out and put a tiny blob of solder on it and then sand it down to fit perfectly between the RX or TX contacts.
The voltage total does not seem to matter, wether 4.8v or 6v to them anyway.
Hope this helps
