I can't figure out why he is absent.

我想不明白他为什么会缺席。

youdao

We can't figure out why he did such a dumb thing.

我们想不通他怎么会干出那样的傻事。

youdao

I can't figure out why he's been behaving so oddly.

我无法理解他为什么一直行为乖僻。

youdao

and you can't figure it out yet you've gone to doctors they don't know why either.

你没法弄明白，即便你去找医生，他们还是找不出原因。

That's why 课堂 - SpeakingMax英语口语达人

With the recent bans on giveaways of formula samples in some hospitals, it's all the more important that the medical community have the tools and knowledge to help mothers breastfeed -- or to figure out why they can't.

CNN: Is the medical community failing breast-feeding moms?