Ah, well my two solutions are these:
First, if you've been playing for a couple of hours, take a short 5-15 minute break. This allows them time to process all the information you've provided, and allow them to think about what's next. When resuming, it may take a few minutes to become immersed again, but from my experience it should help them focus better afterward.
Second, if your players seem bored/uninterested, tell them what they need to know for the story as brief as possible, and move on. This is occasionally a problem in my self-created adventures, and despite the work that's gone into it, if you move on the players might find a subject that they latch on out of interest again, and it's more fun for all.
I hope this helped a bit more...