Why can I not type certain upper case accented letters using the option key with Turkish input? My macOS is configured with Turkish Q - Legacy input source.  To get accented letters, typically one would type Option + &lt;key&gt; to get the accented version of it, e.g., Option + C to type ç or Option + Shift + C to get Ç.
However, this Option + Shift combination for upper case letters DOES NOT work for some letters, even though the lower case version works just fine.  Also, I can press and hold Shift + &lt;key&gt; to get the accented letter mini menu to appear on top of the typed letter.  This, thought, is a major typing speed impediment.
What can I do to be able to type all the upper case accented Turkish letters on my keyboard?
Examples where upper case accented letters CAN be typed with Option + Shift:
Ç
Ğ
Ö
Ü
Examples where upper case accented letters CANNOT be typed with Option + Shift; no output is produced in the app/document:
İ
Ş
Examples where even Press+Hold+&lt;key&gt; does not bring up mini menu above letter:
İ
Now all of the above works for the lower case version of the same letters, i.e., hold Option and type s to get ş.
Thanks to tips from @TomGewecke, I was able to find the app/shortcut key combo that took over the letters I was trying to type.
Amethyst (a great window management tool) uses Option + Shift + I to &quot;Display current layout&quot; and Option + Shift + S to &quot;Select Wide layout&quot;.

I don't have any use for those functions, so I removed the corresponding shortcuts.
For future reference, I am wondering if there is a way to ask macOS for &quot;the list of currently assigned shortcut keys and what app has defined them&quot;.