+1. I have noticed that not all primer pockets are of the same depth - whether this is due to different manufacture or pockets being reamed/modified by reloaders. Also, not all primers are the same height, so proper seating depth may vary -
http://www.thehighroad.org/showthread.php?p=3936874#post3936874
If you hand prime, you can feel the primer feet hit the bottom of the pocket (first resistance) and as you press deeper, anvil being set against the priming compound while anvil feet ride up inside the primer cup (second resistance). I usually seat the primer .004" below flush, but depending on the depth of the primer pocket, properly seated primer may be deeper. Due to this reason, seating the primer flush may not ignite the primer on the first strike.
If you press prime, it is harder to feel for this two stage resistance. Presses that have primer seat depth adjustment may not address this issue 100% due to different pocket depths. When I am press priming, I load the finished rounds bottom side up in ammo box/tray and run my finger over the primers to see if they are below flush. If I feel any flush/high primers, I will reseat the primer deeper in the press or the hand priming tool.
Of course, some have posted that as long as your reloads go bang you are fine, but if you pick up mixed range brass with unknown usage life/primer pocket depths, YMMV. If your primer fires on the second or subsequent strikes, it is due to improperly seated primer. Here's a thread that addresses properly seating primers -
http://www.thehighroad.org/showthread.php?t=568838