Clicking on a critique affects the arrow keys
In the ICM program, when you are displaying images, you might also display critiques. If the critique does not fit on the screen, you might try to scroll the critique display. That action will disable the arrow keys on that image because the critique panel was to process the arrow keys instead of the rest of the page displayed.
Instead of using the arrow keys, if you use "n" for advancing and "p" for the prior image, images will advance properly, even though the arrow keys are not responding. "k" can also toggle the critique on and off, which will also allow the arrow keys to work again.
Once you are off the problem image, the arrow keys will again work as expected.