your heart isn't in it

If your heart isn't in it, you do not feel interested or enthusiastic about something.

心思不在…上面，对…不感兴趣

I tried to look interested, but my heart wasn't in it.

我试图装出感兴趣的样子，可是我的心思完全不在这上面。

We could tell that Sally's heart wasn't in it when she tried singing jazz, so we said she should stick to pop songs.

我们看出来萨里在尝试唱爵士乐时心不在焉，所以我们认为她应该去坚持流行歌曲。