Camera busy after each shot.

during shooting of a wedding last weekend, my XSI locked up after practically every shot. sometimes it said busy. I do not shoot in RAW. i tried changing the size of the quality to see if that would tie up the card for less amount of time......but nothing helped. i had charged the battery overnight as i always do before a scheduled shoot. This happened while shooting a concert in darker situations a couple of weeks before, but assumed it was because of the low light situation. Please help as this is a helpless feeling, plus missing out on great photo ops.
thank you