This is a reply I got from Arriva earlier in the week and I questioned about the cancellations.

"Regrettably, the operational difficulties that we are facing are in regards to driver sickness and shortages. This is having a large impact at the depot, who are doing all they can to address the situation. As some of these difficulties arise with out much warning and we can't predict which service will be affected, it isn't always possible or feasible for us to update the app or our social media with cancellations in time for customers.

Although I cannot promise any short-term resolution to this problem, we are hopeful that these staffing issues will be resolved with in the next couple months. Please be assured that our scheduling teams are closely monitoring the situation and making changes wherever possible. We are recruiting more drivers and we appreciate your patience while we move through the motions of completing their onboarding and training."