For our FedEx Ground delivery I had a package delayed more than a week, and it was at the depot and every day tracking said it was on truck for delivery, but at about 3 in the afternoon it would change to an exception and no delivery attempt made, delivery tomorrow. I called and the people at the 800 number (who are not at our delivery depot) said it was because of COVID-19 staffing issues. However the next business day it was finally delivered, and when I asked the driver if they were having problems with staffing or if they were overwhelmed with packages because of COVID-19, he laughed and said, no, they were fully staffed, nobody was sick, and their package volume was down. The only problem was that his truck, which is the only one that delivers in our town 50 miles away from the depot, was broken down. He normally drives a 24' box truck. But the first day it broke down, they gave him a 14' box truck, and he had to load it himself from the broken truck. Obviously he had to leave a lot of packages behind in the broken truck. But the people at the depot, instead of unloading the broken truck and getting it fixed, just loaded it again for the next day. So each day he was only able to get the new packages onto the new truck and deliver them, while mine, along with the others at the front of the truck were left behind. He said after I called they dug through and found my package and put it where he could take it. Another week went by before we had another FedEx Ground package, and this time he was driving a rental truck.
So in my case, the problem was stupidity combined with cheapness in not getting the problem of a broken truck dealt with.
UPS, on the other hand, has been delivering ground packages two hours earlier than normal because their volumes have been down so much.