Usually overnight leaks are caused by too little absorbency to last the full night. We suggest trying a booster or overnight insert if you haven't already.
If you have a boy or a tummy sleeper, you may especially need more absorbency near the front of the diaper.
If your diapers are absorbent enough, the next most likely culprit is fit.
The following link can give you a comprehensive look at why your diapers might be leaking:
Also check out here for more overnight help:
https://lil-helper-knowledge-base.helpwise.help/articles/210403-cloth-diapering-overnight
And great resources on the blog: https://lilhelper.ca/blogs/press/what-s-with-all-the-snaps-how-does-a-one-size-cloth-diaper-fit