Quite often, it's worth noting that RRB suppliers ask for a coach and a company accepts the job on the basis of it being a coach, then supplies a service bus. Others may supply unsuitable vehicles. Some coordinators report it as I have heard them doing so but it causes too much agro to take firms off the job and source a replacement bus. Sometimes, I think desperation comes into play as well and with not enough operators on some of the portals, it's difficult to get replacement buses.
CMAC (Avanti) for example, when Chester to Crewe is down, it's all Lancashire operators. Abellio (TFW) even had minibuses in from Birmingham on a Llandudno to Holyhead block. What does that have to tell you about how difficult it is to get vehicles.
The contract is still with Northern so unsure why Abellio were being used.
In my opinion, the issue is coach operators not knowing how to join the portals or being on certain portals but not others so local firms with the best buses and local knowledge, won't always be used.
The operators who are on the job, may not always be the best and supply wrong vehicles, double book and/or not provide drivers with the best information. Many drivers follow sat navs and the signs for the station, not the rail replacement stops. Ideally, the Rail Rep suppliers would mandate the use of an app which they create and has GPS tracking and sat nav stuff built in to the correct stops. Then you are literally giving the drivers the directions and correct stops so no excuse for wrong stops and no excuse for anything to go wrong and it's all tracked for certain so if things go wrong, you can check back.