Wrong fertilizer. The man who invented the process for making nitrogen fertilizer won the Nobel prize. Your fertilizer has none of the most important plant nutrient of all: nitrogen. It may be comparable to a human diet of only fats and sugars and having no protein. How well would you do on that diet?

To add to Ted's comments, once the dahlia is up and growing (at least 6 inches high), I blast them with Miracle-Gro, which some growers will say is bad. Reason being, the percentage of nitrogen is some 27%. (vendors say NOT to give dahlias nitrogen when just breaking the ground and many people think that means NEVER give them nitrogen) Plus, my snooty 'garden store' says using Miracle gro on ANY PLANT is like giving it steroids. We'll I l USED to not use Miracle Gro and my dahlias were not very vibrant. I use it now about every 3-4 weeks and my dahlias are abundant and bright colored.

To further answer you though, it is NOT uncommon for dahlias to reach a certain height and then seem to go into a stupor and stop growing. The height can be 6 inches or 12 inches or 2 feet but it will seem as if the plant has gone into reverse and stopped growing. This is natural. While I do not 'start' my tubers and then re-plant later, I have to think that this alone would set the plant back a few weeks. So give it some M-Gro or at least a 10-10-10 if not 20-10-10 fertilizer and it will likely start shooting up again within a week.

Here is what my 'early' crop looks like from my chair in the yard:

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I've never had my dahlias reach the height of the estimated plant (on paper). It may be my own soil conditions, but they do leaf out pretty well and bloom, so I didn't worry about other conditions.

Also, knowing that it grows from the energy of a tuber, I also figured my tubers are not the optimum size yet, since that is usually some bagged tuber that I always thought was not yet mature.

I will always remember the TV commercial for Miracle Gro that was done by the old character actor James Whitmore. Behind him as he spoke was a patch of dahlias to die for. I have heard of many people over the years using it to bring dahlias out of the doldrums especially in hot weather. Liquid fertilizer is absorbed by the foliage and dahlias have a tough time getting enough fertilizer to their leaves in hot weather. Of course, too much of anything is a bad thing, so follow the instructions and do not over apply. Link to article:
http://www.lovethegarden.com/news/dave-s-miracle-gro-dahlia-delights

Lots of color and growth in garden.

Another..

+1 on the nitrogen, I never met a flower that didn't like at least some.

I tend to apply Miracle Gro to the gardens weekly (Milorganite monthly), although I apply the Miracle Gro at half rate or a little below via my injection feeder.

Between the two, I maintain a grand total of 750 flowering plants in 2,000 square feet, in heavy bloom throughout the season.

Right now, 75 of those are Harlequin dahlia which have been blooming like troopers since late May. One Sky Angel is just coming into bloom now, but will continue heavy and constant bloom through October.

15 new dahlia friends are growing and will bloom in late August; I planted those very, very late.

Most or all plants exceed their specified heights, to the point that my Harlequins are two feet tall right now, and the Sky Angel will reach five to six feet.