Warm welcome to the forum.
In order to get better advice please update your profile and include your County and State.
Grapes are not great grown in a container. If you do grow them in a container you need a very large container - 24".
Plants will drop flowers and fruit when they are stressed - poor soil or too little or too much water.
You need to either plant in the ground or in a MUCH larger pot. Not knowing the variety of grape or your regional conditions it is difficult to recommend a soil mixture.
Pot them up using an all purpose soil mixture. Monitor your watering practice.
Elizabeth - or Your Majesty
Living and growing in Lafayette, La.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant. ~Author Unknown