To miss the chance to say something; to fail to mention or express something you intended to say.
大事な感謝の言葉を言いそびれて、後でとても気になった。
I missed the chance to say the important words of thanks, and it bothered me afterward.
会議で意見を言いそびれたので、メールで補足した。
I failed to voice my opinion during the meeting, so I followed up with an email.
『ありがとう』を言いそびれてしまい、それがずっと心残りだ。
I ended up not saying 'thank you', and it has been a regret ever since.