why the airdrop in my iMac not working with my iPhone?

There are both hardware and software system requirements. Read Mac Basics: AirDrop lets you send files from your Mac to nearby Macs and iOS devices - Apple Support

In order to transfer files between a Mac and and an iPhone, iPad or iPod touch

• your iOS device needs to include a lightning connector
• your iOS device needs iOS 7 or later installed
• your Mac needs to be a 2012 or later model with OS X Yosemite installed
• Your Mac and iOS device both need bluetooth and Wi-Fi turned on. You do not have to be connected to a specific Wi-Fi network.

Let me take a guess, and you have airdrop set to "Contacts Only"?

if so, you need to set the "me" contact on your iPhone for it to work. Took me FOREVER to get that one right. Once you set the "me" contact it should work

Okay - I have tried everyone's suggestions - and my airdrop is STILL not working. I have shut everything off and on. trashed the prefs. etc, etc, - made sure contact are set to share everyone - and still nothing. My phone Iphone 5 - OS 9 does not see my mac - we are on the same network - I am sitting beside it - but also - not contact! Any other thoughts?

What model / Year Mac do you have?