Did you install all the bows into the side clips and then try pulling tight? this may be causing to much friction and not allowing things to move properly. I have always had luck starting with hooking up the wires and then installing one bow at a time into the side clips and pulling the headliner tight each time. When you get to the front all the bows are pretty well tight and one last even pull should be all that's required to get everything to a good starting place to begin final stretching and attachment. Also one last thing I can think of is to make sure you cut enough clearance through the listing pockets where the ends of the bows come through. Not enough and it will cause the headliner to bunch up because it cannot stretch out to the sides which will contribute to the sag you are talking about as well. The bows are also place specific and should be color coded as to where they go. If they are not in the correct position you will have issues. Good luck and hope you get it installed.