Instead of the perfect soft "s" or "c" shape, it is hard marbles. I know that means fiber intake is low.
There's no absolute causal link between fiber intake and hard stools. It is the most important factor, but definitely not the only one, as you've proven yourself by taking fiber supplements. If it were that easy, fiber would be a miracle cure for millions of people suffering from constipation and other related issues.
Drinking plenty of water is very important in combination with taking fiber with no effect. The more fiber you take, the more you should drink. Otherwise the fibers just can't do what they're supposed to do (=making stool softer and easier to pass). That's about the only suggestion I can make for now.
