I hate when I have this problem. It's been a few weeks for me too!
Ok seriously. How long has it been on tap? It might just not be fully carbed. If you've left it under pressure for a week or longer and you're still not getting any head you might be dispensing with too little pressure. Some brews just don't have a lot of head though no matter what you do. You could look at your recipe and see if there is room for something to enhance head retention.