Could you give some examples?
I haven't downloaded many itch games, but the ones I have downloaded have never encountered problems like the one you describe.
It could be a lot of different things, if I have to guess I would think you are downloading files that cannot be used on your mac, for example you downloaded a game for android or maybe gnu/linux.
Perhaps they are patches that you must copy over the base game, which will be a different link.
In some cases, they could be bugs.
It's hard to give you advice if you're not a bit more specific, however, the best thing to do is to comment directly on each game.