The first thing to look at is the receiver antenna wire.
A lot of people getting into R/C underestimate the importance of this, which just looks like a bit of wire hanging out the side of the receiver box.
- Never bundle up the antenna wire
- always have as long a straight run of antenna wire as possible (that's what the antenna tube is for)
- NEVER cut the antenna wire - it's a tuned length
I'd also consider popping open the receiver box (if possible) and check the solder connection on the antenna wire.
(Oh, and one more thing - check your TX batteries)