The buff only applies the first time it is cast. If you target the same creature a second time the buff does not apply again.
//Edited Title - Brigby
Just to clarify, are you returning Prodigious Growth from the Graveyard and then attempting to cast it on the same creature again?
Yup.
There seem to be a lot of buffs that are treated as if they are one time things when they aren’t meant to be…
Yes, this an issue that the team is currently looking into. This problem affects other Aura Spell cards such as Oakenform and Knight’s Pledge as well. I’ll go ahead and add this to the bug list for reference.
Hi Everyone. This should be fixed in the 3.1 update!