Your field is probably already cultivated. The workers don't do work twice. So if you send a worker cultivating and the field is already 100% cultivated he will stop his work on arrival.

Does this happen on an old savegame or are you playing on a new profil ?

Using a new profile, I have found on the whole AI are at best less than useless, incapable of doing anything but causing frustration and annoyance and are a TOTAL step backwards from a few months ago when they could actually plant ANY field you instructed them to do, even if it was badly with some field, given badly is still significantly better than not at all, which is what I have now.

Just try creating a new profile and instructing an AI to plant field 15 or field 13 using the big Class or the Deutz, so far for me in over 20 try’s across 5 newly created profiles they have NOT once succeeded in planting ANY part of field 15 or field 13.

The best they do is either drive slowly down the edge of the field to the other end and complete the task without sowing any bit of the field in the case of F15, or they drive diagonally to the opposing corners field exit very slowly and complete the task without sowing any bit of the field in the case of F13

In which state are the field when you create the task? Are they possibly seeded already?
If they aren't, what state are they in. It seems like the harvested state is recognised as planted state (this is due to harvested maize being just a growth state and not it's own type) and will therefore not be seeded again.
If that's the case we can be pretty sure we found the issue

Did you try plowing or cultivating on the same fields? If yes, did it work or did he stop due to the reason mentioned above?

this is with new profiles, so as per new profile the fields 13 and 15 are in an unplanted cultivated state.

Method:
1: create new profile.
2: do the welcome to mission to get equipment etc.
3: sell ALL equipment and cattle and ALL fields except for F13 & F15
4: buy large Class tractor or the Deutz so you own it, also a 1000kg front weight.
5: permanently hire an employee for the tractor.
6: rent a small tractor and the sprayer for 1 hour.
7: manually spray F13 & F15 after filling the sprayer.
8: rent both parts of the seed drill OR buy them if you go for the Deutz option.
9: assemble tractor, front weight and both parts of the seed drill.
10: drive to the location to fill the seed drill with seed.
11: drive the combination to the farm.
12: instruct your worker to plant field 15 or field 13 using the "task" tab.
13: sit back while they drive to the selected field.
14: savour the frustration of your AI not doing what they were instructed to do.

I suspect it's because of the sprayer. The sprayed state must confuse the worker as well.

Have you tried not to start any mission ? Give in goldencrops in the console and then buy all you want and try to make helpers do the work. Just don't try to cultivate the fields as they already are.

For me fertilizing, plowing, cultivating, sawing, spraying, harvesting worked in this order...

So maybe it has to do with the missions. Needs some more testing.

I will try as per the method, but I will substitute steps 6 & 7 for the slurry tanker.

then I will try the goldencrops method without the first mission (step 2).


EDIT's:
the first try was a fail, as there was employees available to hire/employ in the new profile.


field 15 was a planting fail, doing as per the method in my prior post but substituting steps 6 & 7 for the slurry tanker.


field 13 was a planting fail too, doing as per the method in my prior post but substituting steps 6 & 7 for the slurry tanker.

@b101uk Try turning on the debugger for AI. That might give some information about what it might detect or not.