Young Buck
Bucks will never be able to father a litter if they are not mature to do it. They will probably just sit there and ignore the doe because they will have no urge to do anything to her. Bucks usually mature around 5 months old, and older with larger breeds.

Old Buck
Bucks usually stop producing good sperm around the age of 5. Sometimes some bucks are able to father litters at that age and older, but it is very rare. Bucks mature around 4-5 months of age, for smaller to medium breeds. Larger breeds they mature around 6-8 months old. I have a buck now who is 5 and still fathering litters. If you have a really old buck, and nothing seems to be wrong with the doe, try a younger buck.

Split Penis
Bucks with this may not be able to reproduce. If they do father a litter, it will be a very small litter because the slit in the penis prevents enough sperm to pass through during mating. It is a genetic thing and sometimes there appears to be 2 penis's. When young, it could appear to be a doe, but as it gets around 3-4 months you can clearly see a penis, with a slit down the middle. It almost looks like both sexes. (hermaphrodite.) Males who have this shouldn't be bred because a split penis is a disqualification from any and all shows, and it also re-appears in future generations. These bucks would make the perfect pets.

Heat Problems
When a buck is in heat of over 85 degrees they will not be able to reproduce correctly. This is usually during the summer. The best way to breed during the summer is to bring the buck inside somewhere cool, and keep him there for a few weeks. Then try to breed again. A way to tell if a buck is suffering from this, is if his testicles are shriveled up and sometimes appear to not be there. A good healthy buck able to reproduce will have large full testicles.

Vent Disease/Rabbit Syphilis
The vulva or penis will have a scabby inflamation. This may lead to "scabby nose" if untreated. This is contracted by sexual contact and should be cured before bred to another animal. Penicillin ointment to the genitals is a good treatment. Others use a shot of penicillin till it is gone.