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You need to record a mechanic's lien against the property for labor provided, called a materialman's lien.
Threatening to do this usually gets the GC to pay. It is embarrassing to them and the owner can sue them, so they usually pay.
You have to issue a written notice to the owner, you can see a sample here: http://www.uslegalforms.com/samples/MO/MO-05-09.pdf
You can see the MO lien statute here: http://www.moga.mo.gov/statutes/C400-499/4290000010.HTM
I would call the GC one more time and tell them you are getting ready to send a notice of lien to the owner. This will usually get you paid.
If not, record the lien.