Here is a post from Tim Vandergrift who is the Technical Supervisor from W.E. about this kit.
"Keys to getting a port kit to ferment down all the way are 1) Temperature and 2) stirring.
While the instructions say '65-75F', we have to put that big a spread in to cover off everyone's situation. If we make the spec tighter, we get an awful lot of calls about people who just can't keep their house or even their fermentation area at a more specific temperature.
But the fact of the matter is, if you ensure that the port kit is 75F or even slightly warmer (not as high as 80, though) before you pitch the yeast to it, it will have a much more vigorous start than a kit pitched any cooler, and the vigour of a fermentation at the start is a very good indicator of the thoroughness it will display at the end.
For some folks this means putting the kit on a heat register, wrapping a brew-belt around the box before use, or even immersing the bag in a sink of hot water. However you manage it, it makes a big difference.
Stirring is another key. Yeast has a multi-stage life cycle. The growth, or anabolic phase relies on each individual yeast cell making enough fatty acid esters to bud off daughter cells. They can do this through one of two paths. First, they can use YAN (yeast available nitrogen) to sythesize FAE's and we boost the nutrient levels of YAN as much as we can without altering the flavour profile of the kits (nitrogen tastes bitter-salty).
Second, they can uptake oxygen to make it through another chemical path. If you stir you kit very, very hard--hard enough to whip air into it and oxygenate it--the incorporated air will boost yeast anabolism and speed fermentation towards completion.
That's all well and good for the future, but if you're stuck now, you have bunch of options. First, get your temperature up to 75F. In your post you mention having the belt on, but double-check the actual temperature. Most heat belts will drive a 6-US gallon carboy to 72F, but some are not as hot as others . . .
Second, give the port a very hard stir with a drill-mounted whip. Bring all of the yeast up off of the bottom and stir hard enough to get some air into it. Don't be shy, punish it, and keep it coming for at least two minutes.
Take a gravity reading after warming/stirring, and another one three days later. This should do the job, but if it doesn't you still have options. A starter won't work, because the alcohol content is far too high--any starter you try will simply die when you toss it in. Much better is to take another wine (a 6-gallon kit, any kind, as long as it doesn't have oak or elderflowers in it, and it uses the same kind of yeast that was in the port), get it going at 75F and lots of stirring, and when you rack it on day 5-7 from the primary, leave the yeast behind, and rack your port on top of the yeast bed.
The yeast will be fully conditioned to alcohol, and very hungry. That should knock it down within a few days, as long as it's all warm and gets a good stir.
If you have any questions, go ahead and shoot them through to tim (at) winexpert (dot) com, and we'll be happy to sort them out. The Cocolate Raspberry Port is a fabulous kit, and it gets even better with age. I know you'll enjoy it."