There are 3 main reasons why this happens. A lock should not do this and a shooter should not have to get his flint "just so" to work in a lock.
The tit on the frizzen acts like a cam. That is, as it rotates over the spring, the tension should be very high to hold the pan closed and resist the flint as it first strikes, then it should get to a certain point and flip open and stay open. Friction, geometry, spring strength and balance of strengtht between mainspring and frizzen spring, and timing of the cam are most common problems.
Problem #1: friction
Friction on the pivot screw, friction between the tit and the spring can slow everything and create resistance. Polish and lube.
Problem #2: spring too weak or strong
A weak spring will allow the frizzen to flop open easily but won't be able to keep it from rebounding. The frizzen will flop back and bash the flint quicker than the eye can see. A too strong spring is out of balance with the mainspring and the cock cannot force the frizzen open. This is intertwined with geometry problems.
Problem #3: most common- bad cam timing
Take the lock in hand and release the cock slowly. Let it scrape down the frizzen as you hand pry the frizzen open. Keep the frizzen from doing anything by itself- you control it's opening and closing. Note the angle of the frizzen at exactly the position where the flint clears the bottom of the frizzen. Now take the cock to half cock. Ease up your grip on the fizzen. It it static or does it want to close or open? It is probably static. Now push it a smidgeon further open. Note the pressure needed. Note how far you need to push it to get it to the point where it wants to fly open. This is what timing is all about.
Do the same with the flint bevel the "other way" and note how this changes the angle of the frizzen when the flint clears the base of the frizzen.
The only way to fix "timing" is to change the angles of the cam on the tit of the frizzen that rides on the spring, OR move the apex of the hump on the spring if it's a late lock with such a hump. It takes practice and skill to actually improve timing through judicious stoning of the tit on the frizzen.
All these things are affected by the lock's basic geometry and "parity" of spring strength. They all interact. Some locks have very good geometry and so are less prone to having to be "exactly right" as far as that cam is concerned. Others have to be "spot on" in this area to be reliable. You don't want a lock that takes a lot of fiddling with flints of slightly different length, etc.
