We performed some work for Advanced Detection in April and haven't been able to collect on our billings to them. Trying to figure out the next step we should take in order to file a lien.
You're far too late to file a lien, unfortunately. A lien must be filed within 90 days of last work, which in this case would have been July 29 at the latest. Your only option at this point is to sue Advanced Detection for breach of contract.