Hmm....people talk a lot about "writer's block," but there's not much about "reader's block."
Kill all background noise and interruptions.
Make yourself as comfortable as possible.
Create definite start and end points to the task in hand, so the remit is simple, clear, and not too daunting.
Try a dual-pass approach - in the first session, just sift through the work and get a feel for the "lay of the land." WIth a bit of luck, when you go for the second pass, your mind will have a better idea of what to do and how to do it, especially if there's been an interval of an hour or two between passes. Sometimes it's amazing what the human mind can achieve unconsciously.
If you still keep losing focus, try to discover what it is about the work that is making that happen. Are there particular points in the text that buck your attention? Try to analyse the moment you lose your grip on it, and see if you can explain exactly why it happens. If you can't do a thing, the next best thing is to work out WHY you can't do it. For years, all I would do was beat myself up for failing, and I made little progress until I began to wonder what was really going on in the detail of the thing.
As a last resort, cut the remit down to shorter chunks - see if you can manage just one or two paragraphs.
But I'd also be looking at the feasibility of the remit. If they're just piling tons of boring work onto your shoulders, I've no idea how anybody gets through that. Also, is the stuff explained clearly enough? Sometimes I've practically broken myself trying to fathom scruffily-written teaching material, because I've assumed the material was clear and concise. If a book just doesn't make sense to you, see if you can find an alternative information source that suits your mindset better.