Hi Shari,
Here's my experience with this. First, are the reading comprehension scores from Accelerated Reading program? We found that at early ages very bright kids don't want to read "age-appropriate" books. They were interested in more complex or sophisticated stories. Even if their reading level was tested as high, they can't read long books fast so it would take a long time. they'd forget and lose their place etc. So first I'd try shorter interesting books that were of a "decent" reading level like Roald Dahl, or Stone Fox, or The Whipping Boy, or any good book that was about 100 pages. That way they could actually finish within the week and test before the weekend. If there's still a problem, try what we did for longer books: I'd read a chapter to DS, then he'd read the next one to himself but have to tell me what happened to "catch me up" before I'd read the next chapter to him. By re-telling what happened, he would reinforce it in his memory. We'd blaze through neat series (Shadow Children, Warriors, Pendragon, etc.) by alternating chapters or re-capping what happened. This was also a great way for us to connect by conversing about "I wonder if so & so is really the traitor..." or
I still do this with DS7 but the older ones got tired of waiting for me to read the next chapter before age 9. Sometimes they'd read their own book and then we'd have another one we read by alternating. It helped them make big A.R. goals but also introduced them to books they probably wouldn't have read on their own--classics or otherwise slow-action books that might have lost their interest.