Why isn't steam coming out of my Perfection brand steam cleaner?

I have used it before, but now I can feel that the water is hot inside the machine but no steam is coming out when I press the release button. I let it cool, emptied it and checked that it was clean. Still no steam after adding water, tightening cap and plugging in for more than 10 minutes. Why won't it steam if the water is appropriately heated?