Amanda D. answered 07/21/19
Great tutor with years of online and in person experience
There are several possible reasons for this. You may just be more accustomed to reading online than reading a physical book, in which case, ebooks and online textbooks may be more helpful for you in the future. It may be because textbooks tend to have a lot of distractions. Rather than presenting you with a wall of text, they tend to try to include color coded titles, bolded vocabulary words, and pictures, graphs, or examples in the margins to try to get their point across and keep things interesting. Unfortunately, this can be counterproductive, as these may become distracting. One way to work around this would be to cover those pictures and graphs with post-it notes or scrap paper to block them out so you can focus on the main text, then go back and look at them later, after you've finished the section or chapter.
However, another explanation is that textbooks tend to be more in-depth than Quora, newspapers, and many websites, and they tend to use more specific, niche, academic language and technical terms. There isn't really an easy fix for this. If your textbook includes an outline or list of main ideas before the chapter, a summary after the chapter, or a list of review questions, it may be helpful to read these first, then read the actual chapter. This way, you have an overview of what you should be learning, and it will be easier to follow along. You may also want to look at all of the vocabulary and key-terms first, so these don't confuse you when you come across them. It might also be helpful to highlight and take notes in your own words while you read, so you can reread highlighted sections and review your notes to better understand the chapter or section. If you've taken these steps and you're still struggling, you may want to reach out to your teacher, professor, or TA, and find out when their office hours are, as they may be able to clarify things for you or answer questions they might not have time for in class. You may also want to find a tutor to help you go over the information and better understand what you're reading.