I had the same experience. First 1-2 feet were easy.
For a while, I kept pulling back and starting again...
Then, I realized that persistence worked... each blow of my 3 pound
sledgehammer moved the rod almost imperceptably. But after a good
workout of about 30 minutes, I finally go the rod all the way down to
the last inch or two.

Believe it or not, I used a 20# hammer. I didn't swing it over my head
like I was driving railroad spikes but only let it drop about 8" to 12" or
so (about the same as the effective drop of a small hammer. When I only
used a 3# hammer to a good approximation absolutely NOTHING happened!


You might consider putting on the ground
clamp before you start with the hammer. After a zillion blows, the
head
might mushroom a little.

How true! I learned the hard way and had to use my grinder to shave
off the mushroom.

There ain't no easy road!

Yup...

I suspect that I hit down rocks on the way down and the rocks had to
shift
or break up before I could progress. If you have a big enough rock
formation down below you are SOL and will have to used some other
"approved"
method like putting the rod in lengthwise.