Hello Katy,
You do not say if you are trying to knit every needle rib or 1x1 rib.
If trying every needle rib use a 2 or 3 ply yarn as 4 ply is a bit too thick and will make it difficult to push the carriage across.
1x1, 2x1, 2x2, or a wider rib should knit OK with 4 ply yarn, make sure you have the appropriate tension setting on both carriages and that you have enough weight on the ribber comb.
Sue
February 2010