Where's my milk shake?
Can't get back into swimming pool.
Can't complete quest.
Chet won't let me get past.
Some quest items go straight into your inventory, like items purchased in a store. Others may need to be picked up from where they were earned/created.
If you can't find a quest item in your inventory, make sure you thoroughly check where you completed the task to create that item. Your quest generated loot drop could be waiting for you!
Quest items continue to exist between chapters. So if you left a disgusting milkshake at Scoops Ahoy in Starcourt Mall, for example, it should be waiting for you no matter what chapter you are in. Just head on back and pick it up!