You probably need to replace that sprinkler. I've encountered it many times. Sometimes the sprinkler object gets corrupted somehow and continues to play the animation of it operating but it's not registering with the nearby plants. If you can't delete the sprinkler in Buy mode, you may need to delete the object. (ctrl-shift-c and enter "testincheatsenabled true" then Shift-click on the sprinkler, choose object, delete.) Then buy a new sprinkler.