Driver Shortage ???

Is this a 1st the agency I work for, is signing up Drivers for guaranteed 4 day shifts at set times the same as core drivers over the weekend, and are being treated the same, the core drivers are only doing overtime, after the agency drivers are unable to cover the shifts, this is in February the slow time, I don’t know the full contract terms, it must be quite good, as all the self employed drivers have signed up.