Such a long service line like Liverpool to Norwich are extremely vulnerable to delay enroute.
You either add in huge periods of recovery time whdih increases overall journey time, or break it into smaller chunks over key sections.
Add in it this particular route being a proper cross country intercity route, which has and is being treated like a regional one adds to the problem.
Ideally you need a fixed formation five car (at least), both going half way meeting at a location in the middle where a cross platform transfer is practicable.
EMR are under huge pressure to reduce costs across the board, units with faults are long standing. There a certain faults like a PA defect are listed under defects that a unit can not leave a depot with the fault outstanding.
So say 158999 has a PA fault on Monday, it’ll often be allocated a diagram where if stays away from Hometown Depot for as long as possible to enable the unit to remain in traffic. As soon as the unit goes onto depot the fault has to be fixed before it can leave again.