Check for lifted tracks on the receiver board (the little roads between components), check antenna wire is on their solid, then check crystals (the 75mhz ones you have wont work) are firmly seated in their sockets tightly and that the batteries are truly charged, as that can cause these exact symptoms

I always slightly bent the crystal legs outwards to guarantee good pin contact with the sockets.

GOOD: (o) (o)
BAD: ( o ) ( o )