Be sure you have the processing option for the G/L Class set to a Y as you see below.
Update General Ledger Class Code Y
Also be sure to use the data selection for which branches you want the item G/L Classes updated with the Item Master value. The Item Location File
(F41021) G/L Classes will also be updated. You can also select only specific items if you choose.
This does work we use this several times throughout the year.