1. I don't get it. 我不懂。

2. catch your drift. 懂你的意思。

3. I don't get what you mean. 我不懂你的意思。

4. I don't grasp the meaning. 我不懂这个意思。

5. I'm not following. 我有点儿不明白，我听不懂你的话。

I don't catch my girlfriend's drift, she always force me to join some social gathering ...I am afraid to see her...