Thumbnails not generated

Hi!

Since upgrading to 3.5.1 I’m having a really strange issue. I’ve defined the thumbnail sizes in the media section like this:

Thumbnail: 200×150
Medium: 300×300
Large: 0x0 (disabled)
Vertical-thumbnail: 200×335 (user defined)

When I upload a big image (ie: 800×1000) all sizes are generated, but if I upload a smaller image (ie: 400×500) no thumbnails are generated. I understand that if the image uploaded is smaller than the thumbnail sizes the thumbnails are not generated. The problem is that WP is not generating thumbnails for images bigger than the defined sizes (such as the mentioned 400×500 one) either.

Any clue about what’s happening?

Thanks