I have the same problem but not with Simroutes but Vroute. The problem is that Simroutes or Vroute don't create a "real" FSX format. Their is some minor changes on the file that block the import function...

Try to make a flight plan with FSX and you'll see. That will work on the 757/767.

Try to do that : Make a flight plan with Simroutes and then, load it with FS and RESAVE it with FSX and tell me if that work.