Are you just modifying Solidworks drawings without modifying the part? Usually not a good idea. Did you check the help files or tutorials? In Solidworks a red line, or a red dimension, or a red centerline, etc means that it is an entity that can no longer find its source or connection on the part. The fix usually involves going back to the part to find out what's going on. It also usually involves blowing the red entity away and replacing it with a new one. If you are just working with Solidworks drawings without working on the parts too you might as well just convert them all to Autocad and be done with it.