We need to see your predesigned template to see why its 'rejecting' the 2nd image.
It may be possibly due to a clipping mask or a layer mask?
Can you take a screenshot of the layers panel and post it here for us to review?
It's doubtful that PS "rejects" your image. If at all, you would get warnings liek e.g. about color space mismatches or differences in color profiles. It's more likely that there is a specific masking or blending technique involved and that fails if more than 2 images are adeded to the stack... As Sudarshan said, screenshots tell a lot...