Couple issues here.
1. You have your masonry sizes set to Medium, which will do half width columns. You’ve then set specific posts to “Small”, which isn’t compatible with half width columns. That’s why your columns are looking a little weird. You’ll want to set those small posts to “Global” inside the masonry metabox.
2. The load more is working for me, but it is quite slow. This is most likely a server issue, as it’s just taking a while to load the posts from the next page. Make sure your images aren’t too big.