Unfortunately, it seems floppy disks are rather prone to grow a sort of mould if they are left for a long time, especially if in a slightly damp atmosphere like an attic.
It seems to vary make to make, some are really bad, the mould actually breaks down the oxide on the surface of the disk. Other makes don't seem to suffer from it at all.
If you put a disk with mould on it into a drive it will often rub some of the mould off onto the head and then the drive won't read anymore disks.
Get enough on the drive head and it will start rubbing the oxide off the surface of other disks.
A head cleaning disk may get the head clean again but put another mouldy disk in and you will start all over again.
If you look at the surface of the disks carefully (slide back the metal cover on 3.5" disks) then you can often see the mould as light grey splodges on the surface of the disk.
Rotate the disk carefully to check the whole surface on both sides before putting the disk in a drive.
Last year i tried to recover data from over 100 5.25" floppy disks containing software for a rare Powertran Cortex computer from 1986 or so.
They had been stored in a garage for many years.
Most of the disks had mould to some degree.
Some makes were so bad that as soon as the disk was put in a drive the oxide just pulled off the disk.
I tried many ways of removing the mould, some of which would horrify computer engineers.
In the end i managed to read about 70% of the disks completely, most of the data from maybe 20% more and the rest were unrecoverable.
I also got through 3 5.25" floppy drives and had to clean the heads manually with cotton buds maybe a dozen times.
On the plus side, i did discover how robust floppy disks and drives can be, the warnings about not touching the surface of the disk seem to be a bit of overkill compared to what i did to some of them.
Jim