I notice your container is sitting on concrete. Annual asters prefer cool, moist soil and are marginally successful in containers because they need cool soil. Once asters blooms are cut the plant will not flower again. Asters are not heavy feeders and too much fertilizer especially a fast acting liquid plant food can burn the blooms. They also need just one inch of water a week so don't overwater. These are tender annuals that will not tolerate any frost. I would snip off the faded blooms, keep the plants cool and see if you can get those buds to open. If not, time to try a more forgiving and adaptable plant in your pot.   Keep growing, Marianne Binetti