selenium can't find element on page code example

Example 1: why I can't find element

Could be several reasons for example 
-Locator changed
-There is an iframe
-Waiting time issue, page is loading slowly
-Element is dynamic, locator cannot find it. Use dynamic locator.
-Page is not fully loaded/opened. This is also waiting time issue
-Page changes and that element does not exist anymore

Example 2: why i can't find element selenium

Could be several reasons for example 
-Locator changed
-There is an iframe
-Waiting time issue, page is loading slowly
-Element is dynamic, locator cannot find it. Use dynamic locator.
-Page is not fully loaded/opened. This is also waiting time issue
-Page changes and that element does not exist anymore