It can get stuck if it is too small. Maybe increase the size of the "peas".. The idea with pea gravel is that Dr. Bowkers studies have shown it increases blood flow and hydraulic flow through the feet and allows the horse to position their feet where they are comfortable.
I believe Dr. Bowker's presentation to the ECIR group on this study is available to download at no charge. I don't have time to find it for you , maybe one of the mods can link.
Dawn Wagstaff and Tipperary
Saline, MI 2003