Mid 60's will carbonate slower due to the lower temp. But it will carb eventually. Also, make sure the sugar is fully distributed in the bottling bucket by gently stirring halfway through bottling. And finally, don't measure your sugar, weigh it. It is a much more precise way. There are software programs that can help you with this in finding an appropriate weight to gallon per style, as well as the influence of temp on the beer.
(~):} Just a little Furthur (~):}