I've found that with an on-target unlock spell, it is sometimes very hard to unlock anything. Even when you are 1 foot away. So I made my own unlock target spell, and instead of unlock on target, I made it be unlock on target in 3 feet. Now it works every time. Move around a little, once you've found that sweet spot on where to aim, remember that for all chests.
That's what I've done too, and once in a blue pair of moons it'll even work on those stupid gates that are so impossibly hard to hit just right.
Also, as mentioned, you have to use the right spell for the right lock. If the lock is Easy, you won't be able to open it with the Open Very Easy spell, you'll need Open Easy or better.
However, the highest grade you can buy is Open Hard, so you'll have to make the Very Hard version yourself. That'll open nearly everything. IIRC it'll require Expert level (75) to cast.
As far as making spells, you'll only need to buy the cheapest version of a spell in order to gain access to the effect. You start off with Restore Health and Fire Damage already, so once you start making spells, there's no reason to buy more spells like those two. Likewise, Open Very Easy will give you access to the effect, so you can later make the Very Hard version for yourself.