Everytime I have ever removed kingpins, the grease is so hard that is a wonder that I could ever get grease in them at all.
KIngpins are not sealed in any real way whatsoever. That is why the Operator's Manual indicates that they need to be greased every 1,000 miles. Greasing them that often will force out the grease that is already in there, eliminating a lot of the road filth from accumulating and ruining the grease.
If you are not able to get grease in there, I recommend that you remove the kingpin and bushings and clean the thoroughly and inspect for excessive wear and reinstall. I'll bet they will take grease after that.
