Some books take time on me. I loved Brave New World, but the introduction chapter and exposition had me drifting a bit, but i stuck with it. It felt a little slow at first for me, but by the end of the book it became clear how important that first chapter was. I don't believe it was too slow. It's a problem I have, or had, I go from book to book without giving myself enough time to think on it. Now after a good book I wait a week and use my free time letting it soak in or going back to reread a part to understand it better.
My best advice is to solder on and just keep reading. Maybe you're not as into the book as you thought you were. Once I get into a book it's hard for me to stop reading, so I push myself when I loose attention. That is if I find the book interesting.
My memory is pretty damn good too. More than once I'd stop mid book for 1-2 months and know right where I was last I read. I might reread the page to recap and go on: No problems.