Its possible that leafing out to quick can perhaps slow stick from rooting but thats only a guess on my part.
I can only relate from experience as i have had them leaf out and get roots as time went on and the leaves kept on growing and opening more as it rooted and plant did fine.
I have had some get leaves and drop as roots started to appear , my guess is that somehow the roots coming out took over the energy and won out and it turned out ok.
I have had some with leaves first and it struggled to get roots only to eventually do nothing and everything just went downhill .
Its a toss up as i have noticed a lot of times the scion as different and not usually the same.
But for me getting scion to root outnumbers a good deal the ones that are to weak to do anything.
Nass sometimes in a cup they take there time appearing but that does not mean the stick does not have a lot of roots they just have not penetrated the soil to see to yet and or are doing a dance going in a circular motion instead of growing out toward the edges eventually they will.
Jason pointed out to the forum members something that is excellent for members to get an idea if theirs is starting to have roots is by grabing stick and giving it a slight twist a real real slight twist and or gently gently pulling upwards to feel any resistance, just try to b carful as to hard of one and you may or may not break a tender root but thats a very good way of telling thats not mentioned very often.
Best Health