There are a couple different things at work in the rectum. There are three Houston's Valves a couple of inches in that things can hang up on, and the puborectal sling that will catch stuff if it's tight (that sounds like it might be the 4" catch that P'Gell is talking about). Higher up there is the curve into the sigmoid colon (also called the pelvic colon), and there are a couple of things that can hang you up there - the first is the bend itself, the second is a weak band of muscle that can function as a sphincter (though not nearly as strong as the anal sphincters).
Some of these barriers can be passed if you relax sufficiently, especially the Houston's valves and the puborectal sling (you can learn to relax these, but like all muscles that you usually don't use consciously (yet can control voluntarily) learning to control them is difficult.
The bend is another matter. Often the muscles "holding" the bend up (connected to the peritoneum) are loose and you can jiggle something around them, but sometimes not - this bend is probably the usual source of the 7" limit for comfort in many people. Playing around can sometimes get things to line up and allow for deeper penetration, but you do need to be very careful when pushing your limits and getting close to them. Sometimes it's better to just enjoy what you can do.
There's a post on my blog, Bottomtalk, that covers length
link
12 inches is pretty extreme - listen to your body and accept that you probably won't be able to take it all unless you are willing to stretch out some of the things supporting your colon. Is it worth it?
