Venture a guess as to why Rmagick / Imagemagick forgets to shrink thumbnail?

I am running Rmagick and Imagemagick just fine on my Windows XP
machine ( thanks Rick ) and all is well in the world.
I've moved the identical source deck to another machine that appears
to have an identical work environment ( pathing to imagemagic ) and
the application saves photos just fine, but it doesn't do the
THUMBNAIL thing? The photo is left in its respective subdirectory of /
images with the correct name ( photo_thumb.jpg ) but it has exactly
the same weight as when it was processed by Rmagick/Imagemagick.

Could someone venture a guess as to where I might look to explore this
mystery?

Thank you,
Kathleen