I haven't used the Resource Manager before so I'm just assuming it's set up like thread prefixes.
If you go to Forums -> Thread prefixes then click on a prefix, it'll open up the prefix edit page. On that page there is a Usable by user groups setting. Assuming there is something similar for resources, maybe the group you're adding them to, in order for them to have permissions to add the resources doesn't also give them the permission to use the prefix? Or maybe the prefix isn't able to be used in the category they're adding to? Again, I'm not sure on the exact process here, so I'm just trying to help the best I can without actually being able to look at it for myself.