Farming zones are not fully uzed

I have two 5x5 farming zones, configured mostly for plump helmet. I have ~10 dwarfs with Farming labour enabled and almost ~150 plump helmet seeds. Years are passing, sometimes I have 0 food, but dwarfs use always the same only 19 tiles:

How to force them to plant seeds to all 50 tiles?

The thing that I missed was that two tiles ({x=4,y=3} and {x=6,y=3}) were InsideLightAbove Ground. The presence of such bad tile can be seen when you configure seasons -- you have dozens of possible seeds to enable instead of two.
So dwarfs were planting seeds column after column from left to right and were stuck on these tiles. Since there is no way to light the tile back, the easiest solution I found is to rebuild the Farm Plot after blocking the bad tile.

Here I've build a Floor at {x=6,y=3} and then have placed 24 tiles large Farm Plot, that now works fine!