It doesn't necessarily mean that I wasn't able to completely follow the conversation.
ご質問ありがとうございます。
「話の流れが全くわからなかった事はない」は英語で「It doesn't necessarily mean that I wasn't able to completely follow the conversation.」と言えます。
「~ない事はない」は英語で「It doesn't necessarily mean that...」を表します。
先生との英会話の例:
Teacher: During our previous lessons, what percentage do you think you were able to understand? (昔はレッスンの何割位理解できてたと思う?)
Student: Possibly 50%. But it doesn't necessarily mean that I wasn't able to completely follow the conversation. (50%くらいかな、でも全く話の流れがわからなかった事はないよ)
ご参考になれば幸いです。