Some tips.

Pass on all the other items you don't need, and wait to roll on the item you need untill all the other pieces are handed out (less people to compete with), then roll on the item you want.
If you still don't get it then run the ARR again.
There's no restriction, you roll an infinite times untill you have the item you wanted.

Or just be patient and run the ARR weekly.

Personally I run the ARR 2-3 times per day (on non raid days) if there's an item I really want.