Try opening the Tasklist Options and clicking Clear Sorting to clear any existing sort order, then click the column you want to sort on. Sorting is cumulative in SPAERC, which means that if you click a second column without clearing the existing sort order, items will be sorted by the first column and then the second column.
Note: You can also sort items in your tasklist by using the Initial Sorting for Items section of the Tasklist Options, then updating the view.