You can't find what you are looking for only when you are looking for it.
"探し物って、探している時に限って見つからないんですよね。"
- You can't find what you are looking for only when you are looking for it.
- I can't seem to find what I'm looking for when I am looking for it.
「探し物って、探している時に限って見つからないものなんですよね。でも忘れた頃に後からひょっこり見つかったりするんですよね。」
- You can't find something you are looking for only when you are looking for it. But when you forget about it, you suddenly just find it.
You can never find things when you're looking for them.
It always seems like you can't find something, especially when you need to find it.
この場合、次のような言い方をすると良いと思います。
ーYou can never find things when you're looking for them.
「探している時には絶対見つからないですよね」
ーIt always seems like you can't find something, especially when you need to find it.
「見つける必要がある時に限って、なかなか見つからないですよね」
ーThen you find it when you least expect it.
「で、忘れた頃に見つかりますね」
ご参考まで！
アメリカ合衆国 
カナダ 
