Is that field placed on a rather steep slope?

Mods will allow you to place fields on hills, but you can have issues if the slope is too steep for pathing.
The field gets built as the "building points" are in corners the builders can reach, but afterwards you have areas inside the fields your farmers can't walk to. It doesn't entirely disable the field, but it keeps them from completing the seeding part of their job in the spring, which messes up everything. As that 1st bit is not done, they then consider no seeding was done, and won't do anything else in the field during that year (just like when you build a field too late in the year to plant stuff in it).

Sometimes just rebuilding the fields one or 2 rows shorter to avoid the steepest part of the hill will be enough, Some areas are just impossible to farm on.

Originally posted by Vrayna:

Is that field placed on a rather steep slope?

Mods will allow you to place fields on hills, but you can have issues if the slope is too steep for pathing.
The field gets built as the "building points" are in corners the builders can reach, but afterwards you have areas inside the fields your farmers can't walk to. It doesn't entirely disable the field, but it keeps them from completing the seeding part of their job in the spring, which messes up everything. As that 1st bit is not done, they then consider no seeding was done, and won't do anything else in the field during that year (just like when you build a field too late in the year to plant stuff in it).

Sometimes just rebuilding the fields one or 2 rows shorter to avoid the steepest part of the hill will be enough, Some areas are just impossible to farm on.

flat ground.

Is it happening on one specific field, or all of them?
Have you build stuff right against the edge of the field?
Farmers can be very picky about their access points to enter the field to work in there. Sometimes just having a house right against the field (or fences or some ground deformed by putting another building nearby) will mess up a whole field.

And as there are quite a few "better field" copies in the Workshop, can you check the publication date of the one you're using? Some stuff made for older versions of Banished might have pretty weird issues for no apparent reason.