I have fought this issue before as well and it is because vesys is trying to keep all of those wires together and thus cannot calculate a route for them all together. You can only break up a multicore so much before vesys gets confused. The only way i know to not confuse it is to cut back the multicore to the initial split and then splice into the wire from there. So you would then have the base multicore and then single wires from that point on.
If you are intent on using the multicore you can setup a a new wire for each color of the multicore. Find your longest run wire color add all of them to schematic but uncheck the include on BOM box for all but the longest run. It will then calculate the lengths based on each individual wire but only have the one multicore once for the longest wire. Its a cheat when your breaking up a multicore if you want to do it that way.