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Make sure that you used a good quality potting soil and a pot with adequate drainage holes.
Keep the soil moist but never soggy.
I would move the pant to a shady location to help reduce the transplnat stress.
Here is a link to refresh on the care requirements of Asters.

https://www.gardeningknowhow.com/ornamental/flowers/asters/growing-asters.htm

Underwatering and heat stress is the most likely culprit in this scenario. Try watering more often or witha greater amount of water and see if that helps. Also check for any visible insects on the plants, just in case it is a pest issue.

Fusarium wilt could potentially match your plants' symptoms. Check the description in this article:

https://extension.psu.edu/pests/plant-diseases/all-fact-sheets/aster-diseases

Here is more information about it:

https://www.gardeningknowhow.com/plant-problems/disease/controlling-fusarium-wilt.htm

This can be quite common, the lower leaves are older, receive less sunlight and are just aging.

If you see yellowing, this could be watering issues; either too much or too little.

Here is a link to refresh you on the care requirements.

https://www.gardeningknowhow.com/ornamental/flowers/asters/growing-asters.htm

Asters can be treated much like mums and pinched several times before 4th of July. Pinching after mid July may delay flowering. Or just cut asters back by one half in early to mid June. Asters can get tall and floppy so pinching or cutting back may reduce the need for staking.

I would leave an inch of stem and the basal rosette of leaves. This helps you find it and if the winter is mild, it may live through. Some professionals are beginning to recommend cutting things down in early spring before growth begins. The plant "self-mulches" this way so there is less heaving of plant crowns during freezing and thawing temps. If the plant has suffered disease or pest problems during the growing season, remove all plant parts before winter.

Fall is a good time to divide and transplant many perennials. Keep watering all the way through fall; don't let the soil dry out. When choosing a spot for it, keep in mind that many asters are tall and the lower leaves turn yellow and fall off by late summer. The exposed stems give an unbalanced, top-heavy look to the plant so something in front like the geranium Roxanne, to hide the bare stems, will look better. I've also read that one can prune asters in June, cutting back by half, to reduce overall height. This delays bloom time by 1-2 weeks.