At a guess, your at a lower level this time than the first time through, and the game only spawns the liches for high level parties. The simplest fix is to install tactics mod, and make sure the 'always toughest dungeon spawn' component is installed. You may or may not want to install the other components, they do make the game much harder, but if you find yourself missing the Liches thats likely what your looking for. Be warned, that theres also a 'slightly smarter mages and liches' component that makes the liches quite troublesome.
You can find the tactics mod at
www.weidu.org