Regarding your lapidary arbor, I use the same type of drum/belt
combination as you and have found that:
There are significant variations in supposedly "identical"
belt sizes between manufacturers. Given the same dimensions of 6
inch diameter belts, one from manufacturer A will fit much more
snugly than one from manufacturer B.
The same holds true of expandable drums. Some brands will
increase diameter at a given RPM much more than others, due to
the type and quality of rubber composition.
If used infrequently, all rubber compounds (including car
tires) will lose elasticity over time. Your drum, if left idle
for a period of years, could well have succumbed to this
The most outstanding values today in diamond belts, in terms
of quality/longevity versus price, are the metal belts by 3M.
They cut superbly. I use mine regularily, and they last a long,
long time. (No connection with the company, etc., etc.). Good