Hi, I had the same problem after following the installation instructions to the letter! The problem was caused by a lack of free play in the lift rod. They say it should be set to the same number as the stirrup, which was 4 in my case, but the rod was too tight, so I set the rod to 5 (one hole up). Problem cured, and dual flush works fine. I think it depends on the height of your cistern. If it's "between two sizes" on the chart you may need to adjust one notch either way.
Peter, August 2009