Hi iggy, I'm sure you have already checked the following:

Tx and tank are set to the same freq. band (i.e. A/A, B/B, C/C)
Tx and tank ant. is tight and/or fully extended.
Tx and tank batt.s have been changed with known good (fully charged) units.
You are not picking up interference from flour. lights and/or other electrical or electronic devices.

The next thing I would look for would be a broken Tx or tank ant. wire. Also check for other broken wires and or bad circuit board joints like Stern mentioned. I lost a plane Tx due to an un-soldered board joint that failed after the board heated up and created an open circuit.[] Other wise I would return the unit to the seller. HTH.