This is tricky. George MacDonald Fraser's Flashman series contains some of my favorite books ever, so the odds are against me really liking a film version. It's not impossible -- I love both the book and the film of The Princess Bride. But you certainly don't get lucky every time.
But I don't think I'm being too harsh here. The main problems with the movie are that it's too slapsticky, and that's not my fault. It's true that a lot of the time I'm muttering that Malcolm McDowell isn't doing Flashman right, but I honestly think that's because it would be funnier if he were less outspokenly craven. The book version of Flashman usually only acts like a coward when there's no other option.