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Interesting. Well MW loves to hug the ground as it propagates. You may want to try running a few ground radials in the direction of the weak areas from the greenhouse. Perhaps that might pull the signal more into that direction..of course this may reduce the range currently in the other directions.
Though a slightly different issue, Carl had a problem where his signal would not travel in one direction up a hill. He discovered that by running a ground radial wire in the direction of the hill (null), the signal followed it and now that null area has a signal.
As I have said many times like a broken record, every site is going to have it’s own unique difficulties regardless of the pile of experience of others in their particular location where some common things may work for them, but not for all situations. It’s good to try those common approaches to find out things. But when you tried them all, the next step is to go beyond the text book approach and begin to experiment and discover. That is the nature of Part 15 and regardless of others level of experience, unless they have experience with your particular location directly, all that experience they bring won’t mean a hill of beans. So with that in mind, and taking into account of all the things you have tried up to date, begin now by expanding and going beyond the basics and begin experimentation to find the solution.
There is something unique on your site that is causing this highly directional signal where it should be uniform, especially off the simple wire antenna. Now you got the same results with a whip directly attached to that wire antenna stub. Take the giant leap for all mankind and begin exploring beyond the realm of simplicity or common approach and you will find that solution.