The wheels on the floor tool get junked up with hair pretty quickly, at least they did for our Dyson. The metal peg that holds them in has grooved teeth to prevent it coming out easily, but it will come out if you get a claw hammer and give it a good yank. I've just done this for ours, and it was clogged up bad.
This claw hammer pic is indicative only, to actually get it off, I had a foot on the main assembly, and both hands on the handle of the hammer, the peg is stuck quite strongly.
When you get the wheel out, it will probably look like this. (this is actually a bit cleaner than it may be, as I'd already removed as much hair as humanly possible with a knife before disassembly) Once you have the wheel out, remove the peg (can just tap it out with a hammer, you may need a punch to get it all the way out. Not having a punch, I used the handle of a drill chuck, but a screwdriver would also work :) )
Give it a wipe clean, the wheel should then rotate freely on the peg. Then put the wheel back on, and drive the peg in. I've left about a millimetre
of play in the wheel so as not to have it stick to the side, and hence not work again. Also, test it in both carpet and wood floor mode before putting away tools, I remember changing modes and finding that all of a sudden the wheels were sticking out an extra few millimetres
Hope that helps
Oh, and never mind the first picture showing the entire head assembly removed, it doesn't have to be. I actually removed it all before realizing
that the wheels would just come off with a hammer.