If you are unable to mass promote your characters above 4.5 stars, please check if you already have a maxed-out copy of the same character you wish to promote. We are aware of this issue and are actively working on releasing a fix in a future update.
As a workaround, you can still promote any characters manually. In the case of 4.5-star characters, you can scrap them, and then the mass promoter will take care of the excess. For maxed-out Ultra characters, while they cannot be scrapped, you can still scrap all extra copies at any rarity.
Thank you for your patience while we work on a solution! For more information on how promotion works in general, please tap on the info button in the Promotion view in-game.
You can also read the "What is Character Promotion and how does it work?" FAQ for more details.