My security Sixes
I have a few Security Sixes (Sixes, to identify the Security- Service- and Speed Six product line from Ruger), bought through the years since thr mid-seventies. None have had difficulty igniting primers in a wide variety of brands of brass.
Weak (either cut or just old, though age is not usually an issue with the Sixes) as Black Knight mentioned is probably the most likely, two other candidates for the cause of light strikes are:
problem with the transfer bar, either foreign material, thinning from peening or being out of position due to parts wear
The firing pin (f.p.) being slowed by gummed lubricant or other problem or the f.p. being too short.
For an expedient test, try this:
Open the cylinder, put your finger on the firing pin hole and pull the trigger. (You will have to manipulate the cylinder latch to free the firing mechanism.) You should feel the firing pin distinctly. Now do the same with another revolver. Do they feel equally intense? If the Sec. Six feels less intense, that would be a clue that points to the gun as a cause.
I suspect a bad batch of ammo which is probably rare and would be apologized for by the factory and rectified somehow as well.
Good luck
Lost Sheep
p.s. Since Bikerdoc and Mr.Revolverguy have had similar issues with Magtech ammo, checking rim thickness would be a good idea. If the cartridge is not held close enough to the breechface. the f.p. may not deliver sufficient impact to the primer. This brings another thought. Is there excessive fore-aft play of your cylinder within the frame?