crops die if they
- have a roof/ building(wall??) very close
- get eaten (basically, mobs attack crops and kill them, and animals like boar are attracted to do so)
- are too close to each other (can say healthy for a bit before saying needs more space, if its really borderline)
- wrong biome (can say healthy for a short time before getting sick)

to counter this, generally,
loft a farm up 5 tiles or so away from your house/buildings (this isnt much, like 1 floor tile gap)
plant every other square using 2x2 floor tiles as a guide
plant in the right soil as above ^^

this may be excessive, but it will work.