Try adjusting your timer just a bit. Like I said, it's hard at the begining but it just requires practice. Wandering npcs do not affect the coin app, however, they do advance the frames so beware of that.
in hgss you can call elm, the joggler or use the roaming pokemon to check the seed you hit. Seeds and delays always come hand to hand so there you'll see what delay you hit. In dpp you can check your seed using the coin app.