Akayfortyseven is actually right. I believe there may be a bug that hasn't yet been addressed in the Japanese versions. Either that, or the everstone trick just doesn't work very well anymore. It has happened to me on at least 4 occasions. Here are the ones I recall:
1. Timid Ditto (everstone attached), with Drifloon (f)
30 Offspring, 2 Timid
2. Gengar(m), with Timid Drifloon (f) everstone attached
60 Offspring, 5 Timid
3. Jolly Ditto (everstone attached), with Gible (f)
30 Offspring, 1 Jolly
4. Salemence (m), with Jolly Gible (F) everstone attached
15 Offspring, 1 Jolly
This issue is seriously pissing me off. Breeding is more annoying than ever if you can't even rely on it. The only time everstone breeding has ever worked properly for me on D/P is when I bred a Ditto (everstone attached) with my ADV Gyarados.