UICollectionView不会在前几个项目上滚动到底部

I've been trying to create a chat interface that I can reuse. I'm almost done with the implementation, but there's something that keeps bugging me about it. If I start loading in messages like in the gif when I first load the interface you can see that after the 4th message there are 3 messages that don't scroll to the bottom. With the 8th being the first one that does finally scroll. This varies according to the screen size. On the iPhone 6s testing device it reaches the 9th message being the one that scrolls.I've been trying to create a chat interface