Now that you mention it, I think most of my dark Belgians don't seem to get much of a head to them. But, is there anything else common to those beers that could also be a factor? Last week's Session on the Brewing Network had an interview with Charlie Bamforth. He stated that some recent experiments show that many types of crystal malt actually decrease head retention, as opposed to increase it (as had been previously reported in the literature). Apparently, there are oxidized fatty acids that are formed during the stewing process when the crystal malt is made and these have a negative impact on head retention. In the case of dark Belgians, are you using Special B or something of that nature in addition to the dark syrup?
And this can be overcome by increasing other foam-positive ingredients in the beer. I know I brewed a small, hoppy dubbel a while back that formed a giant rocky head that would last forever. It had a considerable amount of dark syrup, but also a crapload of hops to offset it. Obviously, hops may not be appropriate in most Belgian beers, but maybe a bit of flaked or torrified wheat could help.