How dedicated? Do they have other tasks they have enabled and might do, or other major distractions? Hauling, for example.
A good way to check might be to check their current job on the units ('u') screen every once in a while and see if they're doing anything other than planting, harvesting, eating, drinking, and sleeping. Economy timewasters like shopping and storing items might be culprits, or mass building/construction removal.
... for that matter, how many farms do you have? If there's several plots above/below ground, your farmers might just not have enough time to fully cover them all, especially if they're not Legendary. You may need to add one or two, even if their plant stacks will be smaller for... probably several years.
If that's not the problem... how far are the fields from the seeds? If it's a long way to bring the seeds over, farmers are more likely to get distracted by other bodily needs on the way. I think that cancels that planting. If the fields are far apart, then a similar problem comes up if the farmers keep deciding to plant at different fields rather than filling them one at a time, or at least a set close together.
With two dwarves and that many seeds, I doubt it's an issue, but I'll ask more experienced players anyway: Can two dwarves get seeds from the same bag/barrel, or does that retrieval task prevent others from getting seeds from there as well?