why can't i boolish modify an already boolish operated mesh?

hi,

i’m trying to cut some drilling holes into a mesh which i already edited with the “boolean difference” function, but it won’t work again on the same mesh.

now i first produced a mesh difference by subtracting a five times spun zylinder from a larger single cylinder mesh, creating pocket holes by not going all the way through. that worked well so far as visible in the enclosed pictures.
but then when i try to drill further thinner cylinder holes all the way through the pocket holes by using the “boolean–> difference” tool again, nothing happens.

to be sure of operating with two true meshes i converted both seperate objects with ctrl-c to single meshes before moving them together to create another “boolean difference”.

and i also selected the five thinner cylinders first in order to make them the meshes which maintain as whole.

does anyone happen to know why no further holes can be cut into already boolish cut pocket holes?

greetings, veyron

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