Hi Stuart, and thank you for your feedback! You are totally right, I need to improve the UX of my game, expectially providing a better explanation of the rules.
All the mechanics are well implemented , letting the player to modify every single shape in all others. To solve your dilemma, the last 2 machines ( 5 and 6 ) just fill or drain a shape, not only for the rectangles! About the desired shape, I should move the slot from left-down to another position where the player better figure out!
So thank you again, when is possibile for me, I will surely improve my game! :D