Reposting here from previous thread. 
It's because they're too watered already. When watering someone else's plants, an outsider can only help it get up to 4 drops. Hers are still almost fully watered, so until they get down further, none of us will be able to help.
When they get to 3.5 drops, someone can help. We'll be able to keep them from going down much farther than that.
A few more things I forgot to mention: You are only able to help the gardens of others a certain number of times per day. After that, you can't help the gardens of others by giving water or sunlight. Also, if a seed has more than four drops you can still click on it, because you may have an option to give it more sunlight. (Bend it over is the only thing that works.) But the bend it over option and giving drops of water are tied together, so it's a combination of them that gets maxed out daily.
I don't think this is connected to when you get your own option to remove the snail daily, which helps speed up your own plants.
Edit: Btw, you can delete your other post. Just go to Edit, then go to More Options, and you'll see an option to send it to the trash. We try to keep that thread clean of all except the gardens that need tending. 