Make a starter with the same yeast. EC-1118 should be able to do it. Once the starter is at high krausen (I'd give it a few hours at least) start adding some of the must to it. I would add at a rate of one cup per hour for 3 or so hours to acclimatise the yeast to the acidity of the must. After this, pitch into your vessel with some more nutrient/energiser and you should be good to go. Before you pitch make sure that you have given it a very good shake to get some oxygen into solution.
Edit: Woops, I didn't see that you've already made a starter. In this case, go to your local brew shop and purchase some pottasium bicarbonate and whatever they have to determine pH (universal indicator, pH strips etc.) then follow the directions to lower the pH. ALSO make sure that the concentrates you used did not have and additives that might be suppressing the yeast. Finally, take a hydrometer reading to see if it has actually started fermentation but you're not seeing any visible signs. Don't rely on airlock activity!