My daughter used to have that problem, I called it colon busters, seriously, such a skinny kid with huge hard shits! Plugged toilets on a daily basis. 
We now make sure that she gets her veggies as much as possible and drinks her fluids! Try drinking V8 juice with every meal at least it tastes better than metamucil or that other fiber shit!
Benefiber and Miralax are colorless, odorless and tasteless and they do not thicken the drink you mix them into. We tried the more veggies and more fluids plan for our daughter for quite some time, but she needed more help than that. My daughter won't drink V8. She hates tomatoes. We initially had to clear out all the poop in her intestines. You would not believe how much poop came out of her, especially considering how tiny she was. She was below the fifth percentile for height and weight then. She never had much of an appetite because she was always so full of poop. She used liquid Senokot for a while and Metamucil fiber wafers, which taste like cookies, then the more veggies and more fluids plan, then Colace and now Miralax.
