Hello,

Dicksonia antarctica need to be minimum 5 years old or more to produce spores, I think.

My one is 5 years old now and don't have spores to this time. In the nursery I work are plants with 10 years and they also procuce no spores. May be potted plants need much longer to produce viable spores.
Cyathea produce spores much faster. I see plants only 4 years old with spores on it.

regard
Athyriana

Here is a link that might be useful: fern-homepage

Plants need to be 40 years plus or more than 4 feet.
Hope that helps

Here is a link that might be useful: D. antartica