If you are struggling to get something to work go back to basics and try and create the most simple example you can think of. Can you set a single nA current at all just using the Set Current Level VI that is in the driver?
I have used the 6220, and although I cannot remember if I went down to nA (I think I did) I also do configure the range before setting the current level. I see the example does this (it sets the range to 2 nA), which is strange then that you are getting it to work in the uA and mA range.
I did not use any of the sweep functions, all my outputs were static so perhaps this is just a limitation of the device (but I doubt it).