I am going to take an educated guess here and say.....
The milk production is stimulated by hormones. Anything from age, health, and diet and many more factors,could make hormone production vary.
If she is an old mare I would think that hormones are tapering off, but I wouldn't expect this to typically occur in a horse under 16-17 years old. if you are talking about a horse that is,say 10 years old, it could be something related to other issues that would cause her body to not to release the needed hormones to stimulate milk production.
Maidens are the most common younger horses to have poor milk production at foaling, at least from what I have seen and read.
This is just my opinion here, and an educated guess. I know with people, a thyroid issue can cause all kinds of havoc with hormone production. It may be worth it to have a vet check her over before you consider rebreeding her.
Good Luck,
Carolyn