Well, simple x-ray scanning shouldn't sterilize seeds, at least not reliably with doses of radiation that "should" be used to examine packages. Gamma irradiation is ionizing, and it definitely "can" sterilize seeds, though I don't know in practice how often its done.
I do know that as a matter of course most domestic mail is NOT irradiated or scanned. Mail send to Federal facilities IS routinely irradiated specifically for the purpose of sterilization after the Anthrax scare ten years ago. I don't believe that most foreign mail is deliberately sterilized with radiation, because such irradiation can be harmful to otherwise benign package contents. It very well may be done routinely to "suspicious" packages, and it wouldn't entirely surprise me that this is a possible cause of some people's trouble in germinating seeds.
Other than that, sometimes seeds don't sprout because:
-They're just not good seeds to begin with. Not all seeds are created equal. If you're breeding seeds indoors and don't use lots of light, they can be small and have a low germination rate. This is particularly true of seeds from the bottom of the plant.
-They're old and/or weren't stored well. If you're buying seeds from a seedbank, but they're not high-turnover (ie its not a popular strain), they may have been sitting around at room temp or more for over a year, reducing germination rates. If at any point during their transport they were exposed to really high temps, again, this could reduce viability.
