PR (Performance Ratings) are an indication of your horses ability. Although not always the case. If you place a horse in the wrong race continually it keeps losing the PR will keep going down.
On the Training page go to the "Ask Your Trainer" tab and see what he says. Try starting your horse off at this mark. the distance and the two types of track conditions. It doesn't always work, but it's often a good guide. You can try it for four or five races and then change the distance or track conditions if you don't succeed.
Also, if you have a Rating 45 and it's not racing well then don't put it in any class races above Rating 0-50 or any handicap races above 0-90.