When you aren't sure where you went wrong you need to learn how to read what the wood has told you along the way.
Before you bend the stave trace the backs profile on your tillering board, a wall etc, this will give you a definite reference, without it you may not see the subtle changes in it's shape as tillering progresses.
So as you bend the stave further and further at some point set will show up. What you need to bear in mind as this happens is the 'set mantra' (TTB's) no set inner limbs, a little mid limb and the rest in the outer limbs.
If you get any set that isn't conforming to this 'rule' then that area is weak. If you get set in one limb and not the other then it is weak. Your bow will have shown more set in the limb that chrysaled BEFORE it actually chrysaled - this would have been your warning to weaken the other limb appropriately. Without the reference of the original profile drawn on a wall then it can be very hard to spot these subtle speakings of the wood. Once you've got your eye in it's quite easy to make perfectly tillered bows just by watching the set. A while ago me and a few mates were camping for a while so I made a bow by this method and didn't watch it bend once I just watched the set very closely. It shoots great, zero shock, fast and (so far!!) the set hasn't increased or changed at all.
Also ash is very tension strong and imo pretty much needs to have its back trapped to take load of the belly. If your board raised a splinter (and wasn't weak in that area) chances are your board wasn't straight grained enough. Edge grain (1/4 sawn) boards are surprisingly difficult to read and even a hint of waviness indicate a pretty severe grain violation.
