This problem seems to be particularly bad with "trilogies" that are in fact not a series of three interconnected books, but in fact three volumes of a single large novel. As a result, the middle volume is all middle, with no beginning or end of its own.
I think this is the problem with C. S. Friedman's latest, Wings of Wrath
A lot of what I'm reading right now feels like "housekeeping," necessary information to get us moved toward the final climax in the third volume, but not really of that much interest of itself. So I'm impatient to get through it and to some more real action, or even just some interesting hints of fascinating secrets of the past -- but not so impatient to keep me reading obsessively.