I am a subcontractor in Illinois, and we have not been fully paid from the GC for a fire alarm system upgrade at 77 W Washington St in Chicago. We have been trying to work with the GC to get paid on a partial retainage from previous work and a change order to add stairwell door unlocking based on fire department comments. Our job was completed in June 2019. The GC told us that the money is owed to us by the GC (We have written documentation), we still have not been paid. Since we are past the 90-day window, what would be our best legal remedy to either file an LOI or proceed with filing the Mechanics Lien? I appreciate your assistance with this matter.