Driver shortages have become a regular occurrence recently. They're reliant on overtime, so when 1/2 drivers say no or go sick/on holiday this is the result.
According to a new FAQ they've published on their website, they're increasing driver numbers by 17% as agreed with ASLEF. However the end result is still several months away.
But either way it's very annoying and has been ongoing for some time. I've been promised an update in the next Passenger Panel meeting in September.