Where are the skins, blocks, & items?

Skins, both custom and imported, are disappearing or just not loading. Blocks and items are doing the same. Whether it is vanilla or add-ons(modded). There are some blocks and items, mainly wood types and plants/crops, that are only visible. When the player is holding them. Add-ons(mods) that are mainly affected by this bug. Are, volumes are plenty, biomes+, ect.