Your theory that transferring into another fermentor every week produces a clearer beer may very well be true, but it is doing so by reducing the amount of yeast each time you transfer. This probably explains a good part of not only your problem with carbonation, but the relatively low attenuation as well.
You likely did your initial transfer before active fermentation was complete and by greatly reducing the amount of yeast slowed the process considerably. High gravity beers require a considerably larger number of healthy yeast cells than do normal gravity beers, so this premature transfer was not a good thing for fermentation/attenuation.
The second transfer even further reduced the amount of yeast again, likely before the now slower than normal fermentation had completed. Between the reduced numbers and the high alcohol concentration, the remaining yeasts will take considerably longer to carbonate your beer if they are up to the task at all.
Many experienced home brewers, myself included, don't generally rack to secondary. This allows primary fermentation to complete and the yeasts to clean up the off-flavors they created, floculate and settle out. In the case of a high gravity beer I would go 3 weeks in primary and transfer into a glass carboy for bulk aging, only after gravity readings had stabilized at a reasonable level..
Sorry I can't offer anything more, but you didn't post any specific information about the actual ingredients, volumes or your brewing process. I would advise you to read John Palmer's latest edition of How To Brew and other texts on the brewing process and avoid high gravity beers until you master the process.
