One of the things that I cannot stand is repetition. Doing the same thing over and over to achieve some sort of goal. Also known as grinding in gamer terms.
When it comes to writing, there can come a point where you grow tired of writing and you struggle to continue writing. The only situation where this has happened to me is when the scene I'm writing (or even the whole story) is boring. Coma inducing.
It is very important for you to realise this. It's sort of like writer's block which works on the same principle.
Orson Scott Card (I know I quote him a lot, but he's a great writer, so he gets to be quoted a lot) said “Writer's block is a gift from God.”
When things become slow or even come to a complete standstill, it is not your interest that is fading, but rather your story calling for help.
When the writing is coming slow, it is probably because the part you're writing is boring. This can simply be a scene you should cut, or it can mean that you need to plan less and/or improvise more.
When writing comes to a complete standstill, it probably means something in your plot isn't working out. You just have to pinpoint it.
Like the say about chores, you have to make it fun. (Not that writing is a chore, eh?) If the scene is boring for you, it will be boring for the reader. Either find a way to spice things up or cut out the scene completely.
Boring writing leads to a bored writer. So to make your writing more interesting, keep the following in mind:
If the scene does nothing to progress the plot or develop a character, it does not belong in your story.
If the scene does not contain conflict, it will not keep the reader interested.
If the characters in a scene does not achieve something (be it something as small as getting a person's name or something as big as defeating an enemy), and do not want something (be it as small as a glass of water), the scene will not achieve a purpose.
Then again, there can be cases of burnout or something similar. Everyone needs a break sometimes. But that is a discussion for another time.
The important part is to check your writing next time you are not in the mood to write. Do you find it boring?
Here is some good advice that should keep you writing at every opportunity : Write the book you would want to read.