I tried adding a "Item Info Item Collection Filter" onto my main character and onto the Item Shape Grid, in order to hide an item in the inventory when it is being equipped. However, nothing happens when I equip the weapon.
What do you mean by it nothing happens?
The item does not get equipped? Or the filter does not hide the equipped item?
You will need to show me some screen shot or a small video (use streamble.com) so that I can get a better idea of how you've set things up.
If I understand correctly you can an ItemShapeGrid for your main ItemCollection and seperatly you have an equipment itemcollection that you do not want to show in the ItemShapeGrid.
If that is correct, you'll want to have an ItemShapeGrid Data that only monitors your main itemcollection (you can set its name in the list of ItemCollections). Then you might also want to set your ItemShapeGridData or Controller to allow items that do not fit in the grid to still be added OR you can add another ItemShapeGridData for your equipment items, and just ensure the Size is big enough to allocate item in all slots.
I hope that helps point you in the correct direction