It could be a number of things...
1. Your birds may not be a male and female. Have you gotten them DNA sexed? This can be done by sending either a feather or blood sample to companies that determine the gender.
2. If your birds are for sure male and female, they may not have successfully mated. Have you observed them mating? The male may be inexperienced in 'getting the job done' and therefore may have failed to successfully fertilize the female.
3. Your birds may not be sitting on the eggs for the incubation period (I'm not experienced with IRNs, but for cockatiels they have to sit on the eggs for roughly 21 days before they hatch).
4. The birds may be too young to reproduce successfully. Usually birds need to be sexually mature (for smaller birds, around the 2 years of age range) and sometimes it can take a clutch or two to get the mechanics right.
5. Poor diet. Birds enjoy fruits and veggies and the birds should be getting extra food (especially calcium rich leafy greens, cuttlebone) to help replace the nutrients the female ahs lost for laying the eggs. If they have a poor diet (not clean water, getting fed only seeds, which is not good for them to have all the time) they could very well impact their reproduction rates.
Hope this helps you!