What version of Poser?

Check your previews that are not showing, if they are rsr files convert them to png. Search google if you do not know how.

Poser pose directory contains MAT Poses, Morph injections and other pose files. MAT Poses have been replaced with Material Collections since Poser 6, but many users are too lazy or prefer to still clutter the pose directory with MAT Poses. You can change them with little effort to Material Collections and move them. Here is a product that does it for you http://www.contentparadise.com/productDetails.aspx?id=19651

Also DAZ Studio until Version 4 refused to see the Materials folder in Poser runtimes.

Again GIYF (Google is your friend)

Cheers
Randy