At Clementine, we've designed our bonnets to be larger, with adjustable straps to size down. This helps us to achieve a 92% fit certainty. If you're having any issues, here's some tips to help you out.
The adjustable strap
Have you tried using the adjustable strap in different ways to tie a knot or bow once you have it settled on the head? We find that some customers like to tie it first before they put it on, while others prefer to tie it once it's on.
An instructional video