Received a stop payment notice from our subcontractor's subcontractor's failure to pay their union. What do we do?

We are GC on a public works project. We hired a subcontractor, and they subbed out their work to a contractor. We received a stop payment notice from the local electricians union that their subcontractor did not pay the monthly dues. What do we do? We would like to pay the union the dues and subtractor that from our subcontractor's payment, however, we aren't familiar with this process.