Reply to itsallaboutmygarden: I always leave allium heads to develop, especially on beautiful varieties like Allium christophii. They look wonderful for months and months. They also produce seed that can be allowed to fall and self-sow, or collect and sow to raise new plants.
Reply to Alfreder: I think you've answered your own question, and I'm sure teh cooler conditions and low light levels have simply stalled growth. As soon as conditions warm-up, and brighten up, your plants should start putting on plenty of new growth.
Both temperature and light levels play such an important role in controlling plant growth.