Are you actually using the pH down? In soil, typically, you don't need to. I'm guessing that your soil is the problem. If there's no fert to it, this could be a problem. Like usual, I'm going to suggest you get a bottle, preferable the small one, about 8-9 ounces if you can find it, of Dynagro 7-9-5. It doesn't take much for a soil grow of this fert to work. Just one half teaspoon in a gallon of water will be fine, just don't give a whole lot of water, and see what happens. It could be that there is no fert to this kind of potting soil. After two weeks, this plant should be bigger, like 8 inches in soil grow. If you want really fast growth, then learn how to make a DWC 5 gallon bucket. Or ask at a grow store to show you how. It's easy and Dynagro works good for that too. Just follow the directions on the bottle.