I'm currently studying too (9/10 test date, just started). I'm having a some of trouble with the inference/reasonably conclude passages too. When you think about it, it's main idea thing (plus tone on top of that). One thing I've found helpful is looking back over passages before checking answers. Without time pressure, it's easier to figure out the main idea and the author's views, and I can often catch mistakes I've made. Then when I look at the answers and score it, I have a better idea of where I went wrong. Right now, I'm still missing those questions sometimes, but it's gotten to the point where I get them right in that time before checking the answers, and it's carrying over into the actual timed passages.
The other thing for inference questions is that having to take multiple inference steps doesn't usually work i.e., if you're thinking "The author thinks X, so he thinks Y, so Z would be true," then you're making too many assumptions. It seems a little obvious, but I've caught myself doing it multiple times.