I had a similar problem with the same model, the issue was the fitting of the flush button, when I took it out of the mounting it flushed fully and easily every time but when it was fixed onto the panel behind the toilet it wouldn't flush fully and sometimes the button wouldn't come fully out. The hole in the panel holding the flush button needs to be slightly bigger than the button so that it is the bracket that holds it in place, not that fact that it's jammed into the hole bored into the panel. I enlarged the hole with a wood file until it was sitting in the hole freely then fixed it back on with the bracket, it has worked perfectly ever since. Hope this helps.
GB
January 2009