>weird because I have done this before
What is the source of those JPG files... and have you tried opening the file in Photoshop and resaving?
Some cameras put extra "metadata" in the file header that confuses Premiere, and the usual solution is to open and save in a photo editor
There is no CODEC required for JPEG, but they must adhere to certain criteria.
They must be RGB.
They cannot be Progressive.
They cannot contain certain Metadata, as John mentions.
If the Color Mode is NOT REG - Open in PS, change the Color Mode to RGB and Save with Baseline Optimized, or better yet, as a PSD.
If the JPEG's came from the Web, they could well be Progressive - Open in PS and Save with Baseline Optimized, or better yet, as a PSD.