I wouldn't say all major couriers are having issue's. This week alone I've had 4 city link vans, 2 parcel force, 2 UPS and 1 DHL van to my address. DPD who I have a few parcels with (not with Scan though) have been nowhere near the place since the 4th December and we literally have no snow at all around here.
Tbh, I preferred City Link when Scan used them, DPD are shockingly bad, always have been, especially in my area from experience over the last few years. We used to have contracts with them and they lasted 3 months as they were appalling.