I've done some weeding and fertilised the ground, but none of my bots can find the soil to sow the seed; there are only a select few square which they can use.

EDIT: Looking into this, it seems to be because each bot follows the same path.  So I have a bot which fertilises, one who sows, one who reaps and one which harvests.  However, this causes a delay if the harvester hasn't cleared the square for the sower, as for some reason, they cannot find another "nearest" soil.

There's a known issue where a Seeder Worker keeps trying to put seed onto a tile that already has a crop on it. This should be fixed in next week's version. Thanks!

Ahh, cool.
I set up another "farm"; the soil was being fertilised with crop on it (and multiple times), and later the bot didn't sow any seed.

I don't know if that is the same issue.