Hi Mat , this might not be the problem but have a read anyhow .

In order to use a sequence of image files , the file name must be expressed in one of the following three formats:
name.#.ext
name.ext.#
name.#



To use a sequence to create an animated file texture, the file extensions must be separated from the basename and, or the extension by periods.
For example, the following works:
test.1.iff
test.0001.iff
test.1
test.0001
test.iff.1
test.iff.0001

But the following would not work:
test1

Make sure you have your periods in the right places as Maya expects to see it before the sequence number and padding and before the jpg extension.