"Pokeberry does not work well on cotton, hemp, linen or other plant fibers"
"It seems to work best on wool, with silk turning out a coral color. The cotton cross ties in my skeins did not absorb any color at all. "
Tips For Pokeberry Dyeing on Wool - Botanical Colors
We're pretty obsessed with pokeberry dyeing right now for dyeing wool, making ink and just squishing them up and dropping them on paper.
botanicalcolors.com
So what you observed might not be out of place. However, you should try again after heating the juice without boiling.
