I am unable to place seeds in these locations. they are all three blocks apart, and 2 blocks together. the picture shows how its set up.
I have tried placing other blocks, breaking the farmland and replacing it, relogging, but the seeds are still not being placed down in those locations.
The light level two blocks above the farmland in those locations are too dark to grow/plant crops. Try putting better lighting and see if the issue persists.