Hello!
It happened to me before and it's very likely because some features are not compatible with Fresco. For example if you take legacy brushes from older versions of photoshop (I am thinking erodible tips etc), they won't show up in Fresco, despite being within an .abr folder.
Other features may include what you see on the left of the brushes panel (F5) in Photoshop.

Most notably all that have to do with transfer, texture etc... that relies on additional assets built within the brush.
If you are able to identify which brushes won't load, you can open them in Photoshop, tweak/remove all the unwanted options. Save and rename in the brush list, and export those 4 as an .abr, (or simply drag and drop into a library to share with Fresco) and they should load.