The roller is not working on the carpet but works on the tile. What could cause that?

Do you have the height setting correct for the type of carpet? Is it hard if you push it over a wooden or tile floor?

My wife has the same issue. A lot of vacuums have very small motors. These can't be buried into the carpet. The brushed are mentioned to skim the tops of the carpet . Try readjusting the carpet hight on the brush head.

The belt is stretched and does not have enough friction to keep the roller running on a carpet surface.

the motor spins around a steel shaft that sticks out on one end, and the belt is driven by that shaft, belts are easily damaged and broken, meaning roller wont spin. Often a burning smell of rubber is noticed after motor shaft spins on stuck belt. cause is always hair./string/something at the three contact points. motor shaft and belt, roller and vacuum body become clogged. become familiar with taking cover off roller/belt, and cleaning Checking moving parts roller/belt come out easily and simple to clean...keep spare belts.

Seems like the bearing on the side of the roller has gone bad. If there is a dirt gone deep inside, it is hard to clean it. It can also go bad if the roller hits the uneven surface like the transition strip between carpet and tile. It can unbalance the roller causing it to go off its axis and get misaligned.
This is the roller for the Hoover Elite Rewind
http://www.ezvacuum.com/hoover-93002124-brushroll-for-u5509-u5511-u5512-elite-rewind-p-3607.html

Simply answer Yes it would be but try and make sure the the old head is clean before dumpin it just in case

The power nozzle has a reset button. Push that and that should fix the issue. You could of sucked up the corner of a piece of carpet causing the brush roller to become jammed. If that doesn't work, there could be a problem with the hose. The hose perhaps isn't providing the power to run the power nozzle.

bissel pro heat x2 does not suction up the cleaning fluid

I know this sounds silly, but it works. Turn it over and wipe all the black residue off the bottom of it (off the plastic on the bottom that rubs on the carpet as you push and pull it). I thought my husband was out of his mind when he told me that was the solution, but it really works.