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At some point Joe, you have to bite the bullet and demand payment.
If they refuse, you will need to sue them or take them to small claims court. A lawyer can help with this, and even do a demand letter for you, before bringing suit. (try www.romingerlegal.com/attorney to find one)
Of course if they are judgment proof, or file bankruptcy you might be out... Finally, since you have presumably added fixtures to real estate, I would have an attorney file a mechanics lien..