If your rig is not casting good the first questions I ask are, what weight rod, tippet size, and was it windy?
Rods 5 and up shoud throw a basic hopper/dropper rig well provided the other factors are in check. I floated last week and had my 4 wt. rigged for this. Also, if your hopper is a 6 or 8, I would use 2-3 feet of 3x to turn the leader over for you. You can drop the nymph with 24" 30" of 4x. Some people are weary of 3x and I see why, however, you often need this for a big fly. If your hopper is a 10, the 4x should turn it.
If windy conditions are your setting, then shorten everything up and if possible, use a heavier rod. I hooked 12 last week and half were hopper and half dropper. A yellow foam stimmiy is a fine hopper imitation and can still double as the popular Stones of the Bow.
Hope this helps!
cheers