Snapshot ID: 964b419b99a94870b48a53da3abe6b90
I have set it up to use rare gems but for the belt it suggest an uncommon gem.
This is WOTLK classic
Arcane is 2nd priority to your Fire spec, and your Fire spec has the gem quality set to Uncommon. Since that is your only waist item it must be shared by both of your setups, so the setup higher in the list gets priority and is locking in the gem on that slot.
Either increase the allowed rarity for your Fire setup, or drag your Arcane setup above your Fire setup to make it a higher priority.