Think about this, when a hood is closed you are over powering the springs. The springs and a bunch of leverage are saying STAY OPEN. So ever time you pull down on the hood to close it you are bending a number of parts of the hood, hinge, and hinge mounting just a tiny bit. Some hoods bend real easy, I have seen hoods back in the seventies on Chevy pickups fold in half just from shutting them because the hood reinforcement wasn't as strong as the springs and resistance built into the design to hold that big old hood open.
So every time you pull down on the hood a little bit of bending is going on, a little bit of giving up is going on, the hood and everything related to that hinge is giving up just a tiny bit. It's going to be different on every hood and every car but there is a little bit of bending with every closing of the hood.
Your car may simply be at a point where the hood or hinge mounting point has flexed that tiny bit that is effectively "rotating" the hinge mounting points forward, thus raising the back of the hood. Think about this, if you bend the mounting points on the hood up at the front bolt and down at the rear bolt it would be like shimming the hood at the rear bolt. It would be doing the same exact thing as if you shimmed that rear bolt, which raises the back of the hood.
Every time the hood is shut the hinge is pushing up at the front bolt saying NO, I AM NOT GOING TO FOLD UP AND LET YOU COME DOWN, the hinge doing this is pushing the mounting point up a little on the hood at the front bolt, and doing the pulling down on the rear bolt.
I know this is an odd abstract way to look at how this hood works. Picture a diving board.
When you jump on the diving board the rear bolts would want to "stretch", the front bolts aren't doing much at all.
The hood is exactly the same way, every time you shut the hood those rear bolt areas are being stretched and the front are being pushed up.
Sooooo, if the hood has been bent a little bit over the hundreds of times it has been closed (or the few times because it is a repro hood that is weaker than the original) what do you do to bend it back?
It could be as simple as opening the hood too far and pushing it up! If you were to push the hood up open too far and look at the mounting points you may see a flex. If you push up a little bit you may very well bend it back because it has weakened. Often this is all that is keeping the hood from closing as far as it once was.
Often opening up the holes on the hinge so it can rotate further back or a shim are your best options. You an bend the hood by pushing it open but this often will only be a temporary fix as it will simply slowly fail back to where it was.
Worn hinge pivot points will also do this, think about every one of those pivot points being worn just a hair, add up all those hairs and you have enough slop that the spring pushes up the hood at the rear. It could also be a sloppy reproduction. The hinge may not seem bad at all because the spring is pushing it all into place. Remove the spring and move it up and down and you may see a sloppiness to it. That sloppy fit could be allowing the spring to push up the rear of the hood.
Lots of things at play here, to over come them you can slot the holes to rotate the hinge back or shim the front hinge to hood bolt, both will bring the back of the hood down.