Such comparisons are rhetorical devices to substitute emotional appeals for rational argument; its a good thing that people tend to dismiss them.
Make the case for what the problem is with what Bush or Obama is doing, fine; after you've made the case use an example of an act of Nazi Germany that is genuinely similar in the relevant respects to make the point that the problem isn't merely theoretical, sure. But directly compare Bush/Obama to Hitler ... well, unless your goal is to provide an emotional boost to people who have already bought into the substance of your complaint, you shouldn't do that, and its quite right that people start tuning you out when you do that, especially when you open with it.
Make the case for what the problem is with what Bush or Obama is doing, fine; after you've made the case use an example of an act of Nazi Germany that is genuinely similar in the relevant respects to make the point that the problem isn't merely theoretical, sure. But directly compare Bush/Obama to Hitler ... well, unless your goal is to provide an emotional boost to people who have already bought into the substance of your complaint, you shouldn't do that, and its quite right that people start tuning you out when you do that, especially when you open with it.