It sounds as though you are dealing with a stuck sparge. This happens a lot if the grain is crushed too fine or if you are not getting a good slow vorleuf.
I would first look at the crush and if it is too fine, make an adjustment. When you vorleuf be sure to go slow and allow the grain bed to settle slowly and do this a couple times, then drain, the grain bed will act as a filter.
The muslin sock may be too fine and causing blockage as well. Sometimes its better to let a little debris through and get a good running then to try and get every bit of grain trapped.
Another thing to look at is the grain bill. If you are doing wheat or flaked, sometimes it pays to add some rice hulls to the bill to keep things loose.