Sharepoint - Item deleting not firing from selecting item

Could you please add some more details about your environment / code?

• Which version of SharePoint do you use (including language and patch level)?
• Is it a dev. environment (where you can debug your code) or production?
• Is it a document library or a list (there are known differences between them)? - Are you sure it is not invoked, or you simply don't see the effect you expect?

I wrote a post about such problem five years ago (see this post), it might help you to track down the issue.

Could you add an extra line at the entry point of the event receiver just to log the event into the ULS logs, comment out any other code in the method, and test it in the dev. environment?

Simply that the debugger does not catch the event does not mean for sure, that the event has not been triggered. The process responsible for the event handling (like IIS) may crash as well before handling the event (IISRESET?), see the post of Hristo Pavlov.