I have seen a similar pattern; I have scored better on the first of 2 LR sections and seen a decline on the second. I think it may be that one LR section is actually harder than the other, and it often is the second one. The other possibility is that the second section is not necessarily harder, but it happens to have more of the question types that I find difficult. Of course mental fatigue could be a factor.
The only advice I can give is to just look at the question types you are missing and see if there is a pattern, then focus on strengthening your skills with those question types. Also, go over wrong answers as well as correct answers to see exactly why you're missing certain questions. Finally--I know this may seem impossible--but recognize that you have been weaker on that section in the past and try to focus and concentrate more on not making mental errors in that section.
I know, not much help, but I tried.