Hi, so when you say 'loads of fiber', what are you eating? Do you know too much soluble fiber will CAUSE constipation? https://www.webmd.com/diet/features/insoluble-soluble-fiber Not all fiber is the same. Things like oatmeal, beans, apples are soluble fiber. Also, found in supplements. Another reason is that although we have the right amount of fiber, we aren't doing the other things that aid in normal BM's. This includes proper hydration. Do you drink a lot? Think of your intestines as a water slide. No water, things don't move. Lots of water, it flows more freely down the pipe. Exercise also helps us have regular and healthy bowel movements. And lastly, I really recommend a probiotic. This can be a pill form or the drinks they make in the dairy aisle. This has really helped my son who can easily suffer chronic constipation. You might try keeping a food journal to track it is worse after certain meals.
Like the above answer, soluble fiber is what speeds up your body to produce a bowel movement. It turns into a gel and pushes everything down and out. INsoluble fiber bulks up your stool and can slow your digestion down, so it is ideal for people with frequent diarrhea but not for people who experience constipation.
Also everyone needs around 2 litres of water per day (this is 4 bottles of water) or else your stool hardens, becomes dry, and can back up.