When you start a new chapter, there will be some material which just seems to spill out. You have a whole load of ideas bottled up, and so it's easy at first to create new content and increase your word count.
But then something happens... It gets harder and harder to write the closer you get to finishing. Why does that happen, and what can you do about it?
Picking the low-hanging fruit
Of all the content you want to put in your thesis, there will be some things you are confident in, some things that are easy. Then there will be things that take a lot more thought; things that you're unsure of, or are are difficult to explain, or require thorough references, or are incomplete.
So if you start with all the easy stuff, eventually and inevitably you will be left with the more difficult things which take more time and thought. It's like picking the fruit from a tree. It's easy at first to take the low-hanging fruit, but then gets harder the more you pick because the remaining fruit is higher up.
The more you work, the more work you create
What makes academic writing unique is the level of supporting detail required for every idea you present. Almost everything you write requires some kind of reference either to previously published work or to some evidence you present as part of your research.
Even a fairly simple, uncontroversial and well-known factual statement may need a reference to support it. So writing that statement creates some extra work if you then have to go looking through the literature to find out where it originally came from. This is often tedious work, so the temptation is to leave yourself a note (insert reference here) to remind yourself to do it later, because you want to carry on writing, creating more content and increasing your word-count.
So after working on a chapter for a while, there will come a point when everything that remains to be done is either difficult new content or tedious detail. You will no longer be able to sit down and write 1000 words in an afternoon. It might feel like writer's block, and you might feel the burning temptation to leave the chapter 70% complete and switch to writing about something else (where you'll be able to take more low-hanging fruit and write fast again).
But that can only work in the short term. The same thing will happen again with the next chapter, and the next, until you have built up a vast amount of unfinished material. Everything that remains is difficult, and it'll be one hell of a fight to get the thesis finished.
The stress-free route to thesis completion
If you fight against the inevitable, you will lose. But if you understand and accept how it works, then you can work in harmony with the task.
When you've gone through the easy phase of generating writing, and you start to slow down naturally, this is a signal that you should change your focus to working on either the finer details or to think about the difficult aspects of what you are trying to communicate.
Go back through what you have written. Edit. Put in the missing details. Put in the references. Take the time to think deeply about what you want to say next.
Try to anticipate what an examiner's questions might be, and address the difficult issues now, while the subject and the ideas are all fresh in your head.
Accept that it's necessary to slow down sometimes and take care over the detail. Be perfectionist about it. Do it well, and finish the section by dealing with all those tiny details before moving on to the next.
If you can complete one section...
Every section of your thesis requires the same basic elements before you can say it's complete.
It will need all the references in place, with the full bibliographic information. It will need editing. It will need formatting to look like the final thesis. Any figures will need to be well designed and properly captioned. It will need to flow from one point to the next without any gaps to complete later.
You will have to do all these things at some point, for every single section of the thesis. You can either do it all at the end, under massive time pressure, or you can do it as you go. It's up to you.
But if you can finish one section, just one section, taking care of all of these details, then you know what's required for the rest of the thesis. And if you can complete one section to a high standard, then you know you can do it for the rest of the thesis.