Since you are new to the PR system I wouldn't worry to much about loosing concentration. Just follow the steps, make sure you are relaxed and that you have established a purpose. Then use the chant as instructed in the course. The chant is ment for preventing mind wondering off. If you feel that you've lost the concentration stop shortly try bringing the tangerine back to the top of your head and then continue. Breathe deeply and evenly - that alone should increase your focus. Maybe you should try making several passes trough the book if they are to thick starting at different places. But the most important thing is not to worry and to play with all the steps to see what works best for you.
I'm no expert to PR yet and I'm sure someone else with much more experience will answer your post as well. I also recommend you to search the forum for themes relating to your problems (try a search for concentration).
Hope this helps a bit.
