I've had the same problem too, but usually when trying to book something months in advance. I've actually found better standard reward availability on shorter notice, i.e., one to two months out. Now, that is only for domestic travel; I haven't tried booking any international rewards yet. But I was able to get my first choice of flights to DEN next month without any problem on the dates I wanted, and am planning a trip to SFO and seem to be having no problem there either.