What can be done after filing of mechanics lien if GC believes it is past the 90 day mark?

I am the subcontractor working on a project. We filed the intent to lien and then followed up with the mechanics lien. The GC had their attorney send a letter stating it has been over 90 days since our last date of work. They have not been able to provide a last day they think we had worked. What else can we do?