On average with 1 affix having a 95% chance of success is 19 successes out of 20 successful attempts. Just because the rate is high doesn't mean it can't fail. I did a upslot today with 6, 95% affixes and was worried one of them would have failed. Just try again! Try to always get 100%! There's a affix (that has to be on the base item) augment guidance which adds a 5% affix chance. Try not to do really risky affixes that you can't afford to fail unless it's just not possible otherwise.
This would have been a bug if you failed with 100% though (it has happened on the jp version of the game a year or 2 before NA released when augment guidance was first implemented in the game), but that has long been since fixed.