Steamed Artichoke Toughness

[d'Anconia]

Can someone let me know what I'm doing wrong if my steamed artichoke leaves (not just the ones on the outside) are ending up a bit too tough? Steaming for too long or too short, or does it mean they aren't ripe enough or too ripe?

[getfit]

Bring water to a boil on high heat. Reduce heat to low and steam, covered, until 2-3 outer leaves are easily pulled from each artichoke (20-30 minutes or more, depending on size). Leave artichokes in the covered pot until ready to eat, to keep them warm.

this is how they should be done.

[d'Anconia]

Gracias. Last artichoke came out well.