Do you have the right dimensions? Looks like it must be a 4-D array. From the help:
montage(I) displays all the frames of a multiframe image array I in a single image object. I can be a sequence of binary, grayscale, or truecolor images. A binary or grayscale image sequence must be an M-by-N-by-1-by-K array. A truecolor image sequence must be an M-by-N-by-3-by-K array.
What does this say: