I have heard that uneven shedding can be caused by some sort of mineral deficiancy....though not sure what mineral exactly it is at the moment.....it escapes me....anyway, you may try giving her a complete mineral block (one that is high in vitamins and minerals, not one that is mostly salt w/ a little bit of minerals), or a top dress loose mineral to supplement her grain. (ADM makes a great one called the quad block, it can be easily broken in to four pieces and given a little at a time, or split between different horses) A skin/coat supplement may also help.
Could also be rain-rot if there are other signs...like sores or little bumps, or hair coming out in clumps with skin particles on them.....if that is the case, then you need to treat it, along with all of your brushes, blankets, etc., as it is caused by bacteria....and can spread to other horses or back to your horse....
A good bath, if you have a place where it is warm would help too.
Best of luck.