Tubals DO/can fail. Usually because (even in cauterized cases) the fallopian tubes will in some cases either heal, or grow new channels. Whee fun. More rarely a released egg can float around and get caught up in the lower half of a healed open tube, bypassing the upper tube all together. It's all quite rare, but it does happen (only a few thousand cases each year, which is a drop in the bucket when you're talking about hundreds of millions of people, but a few thousand is a few thousand). In the office I worked in, typically one woman would get pregnant after her tubal every other year or so.
In all honesty, you're more likely to be premenopausal... or reacting to something in your diet (like a lot of soy)... or a bug (virus, bacteria, some kind of infection). But if you're having sex & still have all the parts, it's also possible that you're pregnant.