What is another alternative if it has been more than 90 days, and still not paid? It seems too late to do a mechanics lien.
You do not state whether you are the general contractor, subcontractor or material supplier, and whether its 90 days from the completion or termination of the entire project, or simply 90 days from your last day of work - so, depending on the answers to all these questions, it may not be too late to file a mechanic's lien. Apart from a mechanic's lien, in the District of Columbia you have 3 years to file a breach of contract action, so that remains an option even after the timeframe to file a mechanic's lien passes.
Good luck!