IDIOMS

punch below one's weight

(chiefly Britain, idiomatic) To achieve or perform at a level lower than should be expected based on one's preparation, attributes, rank, or past accomplishments.

Example: 2001 Sept. 29, "For men who should know better," Telegraph (UK) (retrieved 23 Dec. 2009):
  But the overall effect is flimsy and insubstantial, a gifted writer punching well below his weight.