I recently entered my first mead into a BJCP-judged competition (mostly to get feedback), and all three judges reported a vegetal off-flavor in the mead. What could be the cause of this? The mead was made with raw orange blossom honey with no additives other than yeast booster and nutrients. It was aged for a year in carboys, with occasional (every other month or so) racking for clarification.
It could be from bacterial contamination, old yeast, or from stale ingredients.
BJCP page, Mead Faults, lists some typical causes:
If you are already using a starter, then check sanitation procedures for equipment and also how you sanitize the must.