First off, try putting the transmitter about 1mm (Yes, 1 millimeter) away from the receiver. Make sure that the transmitter (or it's antenna) does not touch any part of the receiver. If your transmitter works now, then chances are that your battery or batteries are dying, or the frequencies dont match.
The best way to transmit is to match the frequencies exactly, and both units should have an antenna with the tips pointing to each other (but not touching). So if your receiver is at 100Mhz and your transmitter is at 200Mhz, chances are, you will hear nothing.
also, try reducing the values of the resistors in your transmitter.