MAT files are created by Matlab and can contain data of any class and shape. Knowing the class of the single images is not sufficient to know, how the movie is store inside the file. The readers cannot guess this detail, but you can check it easily:
Data = load('YourFile.mat')
What do you see? After a load command, we are not talking about importing from a MAT file, but now the inputs are the variables found in this files. And if you post, what this is, we can create a solution.
Usually It is useful if you post the failing code and the corresponding error messages.