Are both spells at college or other towns?
"Bound Bow is an Adept-level Conjuration spell in The Elder Scrolls V: Skyrim. It can be purchased wherever Conjuration spells are sold, so long as the player has a Conjuration skill of 50 or higher."
http://elderscrolls.wikia.com/wiki/Bound_Bow_%28Skyrim%29
Sort of like with higher level weapons not being available until you level up, vendors won't make higher level spells available until you have leveled that school of magic up to the corresponding rank.