Thumbnails not generated in v2.5

I updated to 2.5 and have add issues with thumbnails since.
It seems that most times the thumbnails files: example-100×100.jpg is simply not generated in the wp-content/uploads folder.

I’ve done a few trials with different files and I can’t quite find the connection. Here is what I have observed thus far:
SIZE:
All of my images are under 2000 pixels in height or width and should generate thumbnails (from what I’ve read on here)
NAMING CONVENTION:
On a few instances a file named image1_scan.jpg would not work but image1.jpg would generate a thumbnail. This did not always work.
MODE:
I made a series of JPGs (quality 5) from PSDs of drawings i had scanned. The PSD files were Grayscale 8bits. I could not get these to generate thumbnails. I then changed the PSD mode to RGB 8bits and re-madem my JPGs (quality 5) and this worked.

Playing with the mode is the last thing I tried but I’m not sure if the key laid in making new JPGs.

Does anyone have any insight?