When did you plant them? They need an early start.
"Start artichoke seeds indoors in late winter, under bright florescent lights. The large seeds will germinate within a week at warm room temperatures, and seedlings should be potted into larger containers as they grow. Six weeks before your last frost date, start hardening off seedlings by gradually exposing them to bright sun, wind and cool temperatures.
Set artichoke plants out in prepared beds 3 to 4 weeks before your last frost date. Exposure to cool temperatures below 45 degrees is necessary to trigger flowering. Before transplanting artichokes, enrich each planting hole with a balanced organic fertilizer, and space plants 3 feet apart.
Globe artichokes have average fertility needs, similar to those of tomatoes. In areas where artichokes are grown as perennials, container or bare-rooted plants can be set out in spring. When setting out artichoke seedlings, plant them slightly high, so that the crown is well above the soil line."
http://www.motherearthnews.com/organic-gardening/vegetables/growing-artichokes