I do +5/+5/+1 every time to get my luck maxed ASAP. Actually its more like +2/+2/+1 with my bonuses not stacking. I understand how to get +5 bonuses but not why the bonus disappears on the next level up.
Because it doesn't work logically: it works as intended. Unused bonuses are discarded once you level up. Only skill increases obtained during that level count toward attribute raises at the level up. In short, to get +5/+5/+5 you must get at least 20 minor skills raises and 10 major skill raises *during that level* (ie, before the level up indicator appears). For instance, you get 10 minor skill raises to Alteration, 10 to sneak, 10 to Alchemy and 5 major skill raises each in Armorer and Block. You'll be offered +5's in Will, Agi, Int and Endurance. You can pick only three so 10 of those skill raises are discarded as attribute bonuses and will NOT show up on your following level up.
If you raise Sneak from 5 to 100 during level 1 you'll be offered +5 to Agility at level up. But you'll get no more "credit" towards Agility from those raises in subsequent level ups. This is the only "check" the system provides on raising minor skills.
However skills gained after you see the level up indicator will count toward the following level up. This includes any training points spent after you see the level up indicator. This differs from Morrowind where skill raises gained after seeing the level up indicator did count toward attribute bonuses for that level.
Mara