Hi, I think I can help you get this bugged trophy to pop.
I had over 250 games played on this console (I don't think stats carry over onto other consoles). Like others, I've not had an item drop for what feels like an eternity!
I had 82 of 88 trophies, 2 of the missing being "Stocked" and the platinum, so surely I must have played enough to have unlocked this?
I have googled every variation of "Rocket league" and "Stocked" and I've read every forum post and watched every video on the subject but nothing helped. 😞
Anyway, before I finish off my life story, install Rocket League on a different console, log in to your PSN account and fire the game up. "Stocked" and then the Platinum instantly popped for me.
I know not everyone has access to 2 consoles but I'm sure you must have a friend or relative you trust enough to help you out with this.
I'm sorry if this isn't very coherent, but I've been in a rut about this trophy for ages and I'm just so happy its finally popped!
I genuinely hope this works for you because I know the pain of it slowly becoming a mouldering resentment.