Hi shizue, it is not a bug, it is called RNG. Sometimes you can upgrade your RIB from +0 to +9 with 15 blessing scrolls, sometimes you can't with 50 scrolls. At least lv 30 weapons do not require upgrade items, which lowers the upgrade costs a lot. What I can suggest you is using the guild blacksmiths if you do not have decent bonuses on your weapon.