Hate to butt in here but I would like to say that being a member of ASHI doesn't mean competency. Having been a home inspector in Texas for aprox. 12 years I was sent information to join many times. Back then, they wanted a couple of your completed home inspections to review (not checked up on) and I believe you did get a little training and testing from them. They are better than nothing in states that do not have licensing requirements for inspectors. But since most ASHI inspectors don't have licenses, then the organization really has no control over them except to drop them from the rolls. States that have licensing requirements can fine, suspend or revolk the license. And although these people spend a lot of time and money getting the license, it still doesn't insure competency. As was stated earlier, ask questions, how many inspections they've done, qualifications, etc. Usually, the one that the Realtor recommends isn't the one that you want working for you.
To the knee wall problem. Can you get behind the wall? How long before you moved in was the room finished? Did they remove the crib bracing to install the room? What's the temperature in the room and how's the insulation on the walls? The drooping ceiling could be caused by inadequate nailing or fastening. Depending on whether or not they installed a vapor barrier it could be holding water. Was the ceiling insulated? It could also be caused by too wide a spacing on the ceiling joist, not uncommon in home owner additions. Has the roof been re-roofed recently? Roofers drop bundles of shingles on the roof and sometimes cause broken trusses or rafters. There's just way too many questions to ask to give any real advice, but this should give you somethings to look into. Hope this helps.