Ha! Just (re-)learned a valuable lesson.
30 year old 5.25 floppy disks are gunky!
I've got a 5.25 drive, and the first DDOS disk I tried worked with VCOPY, but after that none of them worked, including the first one again, so I assumed the drive had failed. Today, I saw someone in a vintage computer forum mention about cleaning the heads, which reminded me that that was something you used to have to do... Used a bit of paper with alcohol on to swipe past the heads, and it came out yucky, tried a floppy again, and it worked fine this time. Then, tried another disk - and it wouldn't work until I'd cleaned the heads again.
You can still get 5.25 floppy cleaners, so I may invest in one of those, because I suspect there's accumulated gunk on the disks so the drive will need constant cleaning