Hi.
The test i did was with 'resources Load All' (see image)
i Made a array set to 'material'
and made a folder 'Resources' then another folder inside 'Mats' and created some materials.
it seems to be working on my end.
Make sure the array is set to 'material' and if you did change it is best also to remove the action and place a new version of the action.
Also for the path only place what comes After 'Resources/' and make sure capitals are correct and no spaces at the end.
and set breakpoint after the succes and failure events to see if it failed or not.
if it still does not work, try on a new scene. with only the resource load action and an array