Action scenes are really hard to write: it's not just you. I have two suggestions, things that I've done myself. First, I pay attention to strategies used by authors I like and admire. How do they get action across? What kinds of verbs do they use? What kind of descriptions? (For more on this, see Francine Prose's book Reading Like a Writer.) What gives these scenes a feeling of momentum? What kinds of sentences do they use in the faster scenes? Do they use more modifiers, or fewer? And while keeping plagiarism in mind, note what phrases they use in describing certain kinds of action. It can help guide you as you revise those scenes.
And second, sometimes it helps to get up and act out the scenes as best you can (though I realize this is not always possible when writing fantasy novels). Sometimes the problem is that we're not describing what a human body actually does in that situation. As you act it out, you'll also get ideas for the things you can describe. “Oh, I put my foot here when I do a sword thrust,” for instance. You might also try watching action sequences on TV or in movies (or even observe a martial arts/sword fighting class). How do people tend to fall, on their sides, on their hands, etc.? What sorts of exclamations do they make? Do they wipe sweat away, or do they ignore it? What does a body do when a sword (or hand or foot, etc.) makes contact?
And then keep re-writing those scenes. You'll start to notice that the scenes are tighter and more successful, that the words are getting closer to conveying what you see in your head. In fact, reading them over later will eventually help you see those scenes more clearly, offering new and stronger details.