How long have they been in the bottles and at what temps. Beers can start to carb, in other words be fizzy, but still need to develop the head proteins further.
Watch poindexter's video from my bottling blog. He shows beer where bubbles are starting to form in solution, but head hasn't developed yet.
Video Link: http://www.youtube.com/watch?v=FlBlnTfZ2iw
A lot of new brewers think their beer is fully carbed, but it truth it may still have a couple weeks to go. Coopers drops are notorious for taking long for them to reach full carb. Often a couple weeks longer than the 3 weeks at 70 that most beers need to take.
Also different grains help in head creation. Carapils, Oats, Wheat all can help in head development.