When you enter Crimson Fields, are you trying to immediately go to Grum? If you are, that may be your problem. You have to redo the instance until someone in Crimson Fields send you to fight the troll. Luckily, Grum Foultooth is one of the first fights you encounter in Crimson Fields, so you won't have to redo the entire instanced area. On some wizards, I've had to redo entire areas to complete a school quests. Good luck.
If you have done this, and are still not able to go there, then that sounds like a bug and you should probably report it. :-)