Most airlines release seats for flights about 255 days before the date of departure. Start looking for seats as soon as you can, airlines only release a certain amount of seats to purchase with miles.
Step2
Find the airline's route maps and find alternative routes. Most frequent flyer tickets allow different routings, as long as you don't backtrack.
Step3
Find out if the airline has any partners on which you can redeem your miles. Many airlines are part of larger alliances, Oneworld (BA, AA, etc) and Star Alliance (UA, AC, SQ) are two of the biggest.
Step4
If all else fails, and you can't find any tickets online, try calling an agent at the frequent flyer program. You may have pay a small fee, but they may be able to find you a routing or seat that you couldn't.