Be sure the cable is routed correctly inside the shifter. It's possible to install the cable so the wire does not go into the small hole in the spool but under the spool outside the guide, will exit the shifter in the right place so you don't notice it. If this happens the shifter "works" as the head catches and pulls "correctly" but as the wire is not guided along the spool the friction is very high.
We see a few DIY cable installs come into the shop this way. Typically the person said they fought to install but then suddenly went in easy.
Cosmo - I just removed the original cables in my SRAM Force shifters and replaced them with Yokozuna cables. I've found that the cable head sometimes comes loose from the spool. I have given the cable a good firm pull (as per the instructions) to ensure the head is seated correctly in the spool. But if I run up and down the ratchet, sometimes the cable head comes loose.
Note I haven't tightened the cable to the rear derailleur yet, it's still hanging free once it leaves the housing.
Will this still be a problem once the cable has more tension when it is attached to the derailleur? I'm a bit bemused by the problem as even thought the Yokozuna cable is slightly thicker than the original SRAM cable, the cable heads seem to be identical in size.