you can only sort by Y value for the first dimension, if you want to sort on the second dimension, go to sort tab and click on the desired dimension and sort it by expression putting there the expression that you are using in the chart.
Deepak, instead of putting us to think of all possible reasons and solutions, perhaps, it would be wise if you could share your qvw with sample data that demonstrates the issue.
maybe for the second dimension Sort, uncheck all sort options and then sort it as per expression and in the expression use the expression used in the chart. (Asc or Dsc)
