Clips being cropped when placed behind others?

Hi there,

Apologies in advance if I'm not explaining this very well, I'm very new to this. I have included a screenshot so hopefully that helps.

I'm currently working on something that involves multiple clips appearing over or behind each other within the frame.

I have filmed everything on green screen and composited the clips, but the issue I'm having is that when I try to place a clip behind the initial one, the second clip is essentially "cropped" to the edge of the clip at the front. Would really appreciate some tips of the best way to get around this. Thanks so much in advance and please let me know if I should include any other info!