Hi. Are you sure you have a pair??
There could be a number of reasons why this is happening:
1. Sometimes two females may bond just like a pair, produce eggs, but they will not hatch as they are not fertile.
2. If your fish are a pair (ie: 1 male and 1 female) one of the fish may be infertile.
3. How old are your fish?. Sometimes young fish get it wrong the first few times they spawn, but will succeed after a number of attempts.
4. What are your water parameters? Water PH at the incorrect levels can sometimes affect the hatching rates of eggs.