Does it lack carbonation or just does not foam up when poured? There are number of issues that could lead to this.
1) Lack of Carbonation (Simple Reason) - This would most likely be do to under priming the the beer before bottling. I would recommend a priming calculator to more accurately carb your beers. You can find these at a number of websites and a quick google search will give you what you need.
2) Lack of Head (Complicated Reason)- Generally this results from the inability of a beer to create and retain the foam when CO2 escapes the beer. One reason for this is a lack of proteins. There are 2 major proteins and the names are not important but they create and hold the head of a beer. Another reason, as another poster mentioned, is fats from soap residue. Also stressed yeast can produce protease which will inhibit head formation. This happens when yeast are under pitched or under aerated. Lastly, you can get poor head formation and retention from high fermentation temperatures which creates fusel alcohols, these inhibit head formation.
Hope this helps some!