One thing that may help--not to do with up air itself--is to land the sour hit of dash attack. That way they get sent nearly straight above you and initiating the string is technically easier. On fast fallers, I find it much easier to do FF up air combos at pre-tumble %'s with the 80-degree hitbox of dash attack. The difference between the strong and soft hits of DA in performing the combo is especially noticeable on heavy fast-fallers like Ryu and Falcon, because their combination of weight and fall speed results in them taking very little knockback from up air before tumble %'s, meaning you have to be perfectly precise if you want to get max damage off of up air strings on them.