Observer sometimes does not detect netherwart being planted

I am using observers to determine netherwart growth for an AFK netherwart farm, and about half the time it will not detect when a netherwart is planted. It is very unusual because sometimes it will detect me planting a netherwart, and sometimes it won't. This randomness has happened in the same soul sand block. I have double checked that the conditions are exactly the same for when it detects the planting and when it doesn't.