This one has me confused. I installed a new derailleur (7800), set the limit screws to get it in-line with the small cog without the chain on and shifted to the far gear. Put the chain on and it ran great in the small cog.

Then, I put the cable on, and fine-tuned the tension. Now it wont go down to the small cog. I can loosen the tension the point that there is very noticeable slack and it still wont go down to the small cog. I can see that the limit screw is a millimeter or two away from touching, so I know the limit screw is not interfering.

I took the cable off and it will drop down all the way to the small cog, but if the cable is on, even with tons of slack in the line, it wont. What's up with this?