In PowerPoint you can have separate text boxes, or you can add text directly to shapes (with visible borders or backgrounds). You can symbolise text contained in shapes with visible borders/backgrounds, however, depending on the symbol position option you have chosen, you will see that the location and size of the shape may change when you symbolise. This is necessary to get the symbols in the right position in relation to the text, but it probably won't be what you want. To avoid this problem, use separate text boxes (without visible borders or backgrounds) for any text you want to symbolise.