image upload problem- medium and thumbnail sizes not created

I have referenced the sticky info for image uploader problems and found a Q/A for my case- images are uploading with no errors, and actually appear at the correct size on the page due to the width, height attributes, but the file being pulled is actually the full size version of the file. In the past, Ive been able to see the other version that have been created on upload in the uploads directory. I tried disabling mod security via the htaccess to no avail, and even tried uploading an image file that I had uploaded to another installation where this was working fine. Any ideas?
thanks in advance! -Adam