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