Hi there
It could be, that your IC is not working but before replacing it with another make sure that you have assigned both encoder and decoder the same 8-bit address. Also check out for the valid transmission(VT) pin, while testing. It is the 17th pin of HT12D IC and it gows high each time it receives a valid data from its paired encoder. Further, see whether you have provided this IC with a proper resistor value, between its osc1 and osc2 pins, or not. Check out its datasheet for the resistor value.
Hope this helps 