Declining yields from saved shallot sets

About 10 years ago an elderly gentleman who was too infirm to continue gardening gave me some un-named shallot sets (white, globe type). He died a year or so later so partly out of sentimental reasons, and partly because they produced huge shallots which also kept well I kept saving what I thought were sound bulbs to plant the following year.

Over the years the health and vitality of the crop has steadily diminished to the point where this year there is nothing worth saving to re-plant.

Have successfully grown Mikor for a few years and Vigarmor this year so wondering whether the others somehow ran out of steam or I did not use sufficient care in selecting the right bulbs to re-plant.

Anyone else had this problem over the years?

interesting one from a biological point of view isn't it? Were the original sets genetically identical to each other? If not, it's possible you've unconciously selected the weakest clone to replant each year...but seems unlikely... if they are all one clone, why should they get weaker rather than getting wiped out by one infection? could it be depleting nutrients? not if they're rotated. Build-up of mutations? possible... I think I might go for a build up of mutations or of sub-lethal viruses....

Where's a plant pathologist when you need one?

M