I have 10 Cruelly Barbed Brone Bolts in slot 4. I try to move them to slot 6. They poof back to slot 4. I was in a shop, so I try leaving the shop and doing this - same result. I try putting them into slot 5, and that works. Put them back into slot 4, no problems. Try to put them in slot 6? Nope. I tried putting them in 5, and then 6, nope. Why the shit can't I put these bolts into slot 6, guys? D: Also happens with a Kilt and a Diamond. I think the only way I get things in there is when they're consumables like potions.