I'm a rather tiny fan of The Brady Bunch
and I was just wondering, which movie
was Susan Olsen not in and why didn't
she appear in it?

Susan was not in a very Brady Christmas.

Correct, Julian...and I believe the reason she wasn't in it was because she was on her honeymoon at the time, although she and her ex-hubby are now divorced.

The official reason for Susan's absence, given by Sherwood Schwartz and CBS, was that Susan was on her honeymoon. However, Susan has disputed this in a couple of Brady books. She claims that the budget for Cindy's salary was split between Maureen McCormick and Eve Plumb, because CBS was nervous that they were going to turn down the movie. CBS then told Sherwood that there was no budget for Cindy, and to have her character be "away" in the movie. Sherwood supposedly paid for Cindy himself, and hired someone cheaper than Susan.

Who knows what to believe?

This is a quote from Barry Williams book
Growing up Brady I was a Teenage Greg
"A very Brady Christmas was the biggest success in the history of the Brady Bunch.
We were all laughing & hugging about the film
(the lone Susan Olsen,who was busy honeymooning at Jamaica's "Reggae Sunsplash").