I’ve had a similar problem with my agricultural industry. From my research I’ve found that the issue is a lack of uneducated workers. So I built a residential neighborhood w/o any schools near my farming district, but that didn’t fix it right away either. I think it takes quite a while in game time for all the cims that are educated to grow old and have children who become uneducated, and they grow up to then work in the industry.
Also, my farming district is in the northern tile (relative to my starting tile). I have a theory that it needs to be further away from my city center in order for my farming residents to not have easy access to the school systems.
P.S. Is it elitist to think that a farming industry has no demand for well educated people?