For me the volume directly relates to just a few variables. Difficulty to blow can be attributed to a few different variables as well. A little more information would go a long way. Is the insert one that you've made, or bought? If you made it, it is likely that the things making it quiet and hard to blow are separate issues. The material is also a factor, softer woods have a noticeably softer tone, even soft maple is too quiet to get a real high ring. The easiest way to make an insert louder (in my experience) is to expand the exhaust a little. Just a slight taper will go a long way... conversely, this is also a surefire way to make it harder to blow if you go too big. Other ways to increase volume are a very tight fitting cork, and larger tone channel bore (which also makes it a bit harder to blow). Specific things to look for with blowing difficulty, off the top of my head, are the issues aforementioned, barrel too long, reeds being too long or improperly inserted, and an issue with the radii of your toneboard. I'm sure there are other possibilities, but these are the only ones I've encountered myself.
