Some of the older ammo was made with hard primers.
I once tried a box of Swedish-made .380s that dated from the early 1940s.
My Walther PPK hammer hits fairly hard, but about a third of them required a cocking of the hammer for a second try. They all fired the second time.
You may have found some hard primers, in that old ammo.
The only handgun I own that will fire any primer, no matter how hard, is my 5-year-old Browning Hi-Power in 9mm. The Hi-Power is noted for having a powerful hammer spring. This strong spring also serves to retard the slide a bit when the powder is burning behind the bullet.
You may also have encountered ammunition with contaminated primers. Oil can seep in between the primer and primer pocket, deadening or affecting the primer's reliability. One of the worst offenders for this is WD-40. I stopped using WD-40 in my firearms years ago, not only for this reason but because it becomes a hard, gummy varnish over time, affecting a gun's reliability.
But, if the primer is barely dented as you say, then I'd figure your hammer spring is too light or weak. Some shooters who have bobbed off hammers on their handguns have affected reliability too, because they removed too much weight from the hammer.
A light hammer may equal a light strike on the primer. Most gunsmiths who bob off hammers will install a slightly stronger spring to compensate.
Hope these all give you an idea of what the problem might be.
Check the crimp on those old shells, they may be a 
