There could be a problem with the hen or cock's fertility.
Some factors to consider when dealing with an infertile pair of birds are as follows:
- The ovary may be non-functional because the bird is really a male, too old, too young or ovary is diseased.
- The nest box is unsuitable
- The paired birds are incompatible amd have not bonded
- Not the proper season
- They may not be in breeding condition
- poor diet
- stressful environment.
- perches unsuitable for mating.
- males nails are too sharp for the female to allow mating.
- they are not fertile
Instead of stressing out the two birds it is advisable to give them a break and try again down the road. If it happens again you should probably pair them up with a different mate.clipping the feathers around the female's vent can help in some cases.