Usually, these questions turn out to be pathing issues.
So, to start the troubleshooting:
Do your dwarfs have a valid path from the still to the stockpile your plump helmets are stored in? Check that all stairs are properly formed, that the still is not blocked, that no ramps have been removed, etc.;
Have you forbidden the plump helmets? Sometimes, when mass forbidding items (via d-b-f), players unintentionally cover important items;
Are the plump helmets in transit or in use already? If they have 'TSK' beside their descriptions when you hover your {k} cursor over them, they're otherwise in use. This often happens when they're stored in a barrel, which is in transit to pick up/harvest more of them;
Are dwarfs already eating them? This is a subset of the point above - if a hungry dwarf has already marked a plump helmet for eating, it is unavailable for brewing;
Have they finished growing? If all of your plump helmets are still unripe, your brewer cannot use them.
Xxx
xxx
xxx
This is the layout of the still. The capital X is an impassable tile; if the only access point you've left for your still is through that square, no dwarf can get into/out of the still to collect and brew anything. If you've built walls around your still, try removing one or more to allow access through any of the other 8 tiles.
If none of these questions narrows down the issue, please provide more information. Screenshots are especially helpful.