It happens to me a lot, including in English, though I’m definitely ADHD. It also doesn’t matter how interested I am in what I’m reading. I feel it has lessened some though since my proficiency with Japanese and reading in Japanese has increased—with needing less time to look things up and to think about what I’m reading, I have more of a chance to get completely into what I’m reading and there are fewer side roads and rabbit holes for my brain to go down. But even so, it still took quite a while to notice any real improvement on that front. I’ve been reading for almost 2 years now, and it’s really only in the past few months that I’ve been able to read more than ~10-20 pages of a given novel in a day, even though my comprehension and stamina for reading definitely went up before then.
I second setting small goals. In manga I don’t like leaving off mid-chapter, so I try to just read one chapter (and then sometimes I’ll find I can read then next, and the next… the whole volume. But sometimes just that one’s all I can manage). In novels, I’ll flip ahead to see when the next chapter or break is and try to read to that if it’s close, or at least just to the end of the page. Or the paragraph if it’s particularly bad, but if I’m leaving off at the top of a page or section, it’s easier for me to remember where I was when I pick it up again, which is why I try to make it to there lol. But yeah, our brains like short, clear goals a lot better than long, vague ones, so they can definitely help with focus.
(Music too, though with me it doesn’t matter if there are lyrics, even Japanese lyrics, as long as the song is sufficiently familiar to me. Too new, and my brain’s more likely to jump to that. But if I’ve got a song/album/playlist that can just sink to the background, it gives the back of my brain something to do aside from going off on tangents, though it’s absolutely not fail-proof.)
Sometimes I just need a change of scenery. If I’m in my bedroom and am having trouble concentrating but don’t wanna put it down, I might try moving to the living room or the computer room.
Sometimes, though, it means I need a break, whether that means I switch it out for something else I’m reading (or start something new), or I do something else entirely. Some days I don’t get much reading done at all even if I technically have the time and energy, simply because it’s a bad brain day.