One of the reasons I hear quite often on why someone lost a game is they "Didn't see" something: the bishop that took their queen or the combination their opponent played on them, or the combination they missed that would have won.
But there are many reasons why you didn't see something and, if you want to improve, simply writing it off as "I didn't see it" will not help minimizing your chances to have the same problem recur again and again.
So let's list some of the reasons you might not see something and then briefly discuss what you might do to minimize recurrence:
- Failed to look for ALL the opponent's threats from his/her previous move,
- Failed to look for the opponent's forcing moves (checks, captures, and threats) that he/she could make in reply to your move (common!),
- Failed to look for your checks, captures, and threats,
- Literally did not see the piece that moved before it did (board vision),
- Looked for his checks, captures, and threats but didn't calculate that one correctly (analysis error),
- Played too fast for the situation (another common one), and/or
- Was a tactical pattern I tried to recognize but was unfamiliar (lack of pattern recognition/tactical problem study).
OK, now let's briefly discuss/link what you can do to minimize each from happening:
#1 often occurs when you erroneously ask "Why did he/she make that move?" instead of "What are ALL the things my opponent's move does?" It only takes one reason you miss to lose the game so finding just a reason for the move may be fatal. See The Ways a Move Affects the Board and the chapter "Just Because it is Forced" in my Everyone's Second Chess Book.
#2 is the infamous "Hope Chess" - see The Secrets to Real Chess
#3 is partly covered in Making Chess Simple and many books on how to spot tactics like Hertan's Power Chess for Kids or my Back to Basics: Tactics.
#4 You can improve board vision many ways - see The Amazing Power of Board Vision
#5 A big subject. Some of my articles include Bootstrapping Analysis Skills, Analysis Insights, Quiescence Errors, and Analysis Tips plus included references.
#6 - See Real Chess, Time Management, and Care, Putting it All Together, and Slowing Down.
#7 - see Tactical Sets and Goals and its included links
To summarize, if you can break down "didn't see" to the root cause, that often helps you decide how you can work to minimize that problem in the future. Good luck!