Carbonation too Light

I brewed my first Russian Imperial Stout and bottled it 3 weeks ago. I kept it in an area with a steady temp of 68 degrees. Opened the first one today. The flavor was amazing but there was very little carbonation. Any tips on what I can do to increase the carbonation?

An Imperial will take time. If you had a hint of CO2 escaping from this first bottle the yeast is working on the priming sugar. Try another bottle in four weeks. Chill this bottle for at least two days before opening. Carbonation will take slightly less time with an Imperial if you can have the bottles in the low to mid 70°F temperature range.