Some fleeces have particularly muddy tips. There are some extra things you can try if a regular wash just isn't working.
1) pre-soak the fleece, as long as overnight or as short as an hour. I just put it in a big bin with tap-temperature water, since it's going to cool off in that amount of time anyway. I've heard adding salt can help, but I usually don't add anything.
2a) add some denatured alcohol in the wash with the wool scour (this helps loosen lanolin, if the mud is sticking due to that). You'll find d.a. in the paint section of your hardware store; or
2b) wash with Fiber Master, it's good at more persistent dirt
3) after washing and drying the fleece, run it through the picker and wash it again (or just the muddy parts)
Other things I've done:
* skirt the fleece more to take out the worst of the muddy tips
* once the washed fleece is dry, flick the tips open of just the muddy ones and decide if they need to be washed again or not (sometimes lanolin and more muck hides under the mud, sometimes not)
* while the fleece is in the hot wash bath, put on some really thick rubber gloves and massage the muddy tips open. If you gently rub them while keeping them entirely submerged, they should open up and release their mud. Margaret Stove discusses this technique in her book (out of print now) Handspinning, Dyeing and Working With Merino and Superfine Wools.
I've yet to find a good silver bullet, other than skirting away the problematic muddy-tipped parts of the fleece. But that only works if you don't mind ending up with less fiber. I'm more likely to pick or flick open the offending parts and re-wash them.
See here for the rest of how to wash raw fleece.
(based on a post by me on spintoknitsocks, this day)