If the clay is dry, it will be hard and a regular blunt tip will just spin around and around. You need to do one of the following:
1.) Moisten the soil in the places you want to dig. Saturate it enough for the water to seep the full depth you want to go. Then wait a couple of days for the moisture level to decrease. If you dig too soon, it will be a mucky soup. There is a point with clay at which the moisture content is just right and it is very easy to dig. Just a little too much water, the hole just keeps filling with mud. Just a little to dry, and it is too hard.
2.) Use a fishtail tip, preferably carbide, and downpressure. Downpressure can be done with a hydraulic downpressure attachment, or by adding ballast to the top of the PHD frame. I have several posts and pictures of my custom modifications (using this technique) in the archives.