Macbook Pro not going to sleep after lid is closed

Does this happen when its plugged into power or not?  Or both?

What is the macbook model and current OSX?

MacBook Pro (13-inch, 2017, Two Thunderbolt 3 ports).  MaxOS Majove.
I think it happens in both scenarios but cant be too sure.

Reset the SMC and Reset the PRAM.  That's generally a first step to clear "weirdness" and unusual behavior that didn't previously exist.

Thank you.

Please try what serialband has suggested and report back on if the issue persists or not.

How do I do that?
Can you confirm it won't reset anything else or delete anything?

How do I do that?
Apple explain both here
PRAM - https://support.apple.com/en-us/HT204063
SMC  -  https://support.apple.com/en-us/HT201295

Can you confirm it won't reset anything else or delete anything?
Correct .. it has NO effect on the Hard Drive and any data stored there related to your user accounts or applications.

If resetting the PRAM and SMC doesn't fix it .. then we move to a hardware issue.

The sensor that detects if the lid is closed is in the front left corner of the machine and is part of the battery indicator.

A trick to test is to take a small magnet (like a fridge magnet) and run it along the front left edge of the machine above the battery indicator. If the machine doesn't sleep the sensor may need to be replaced.  It may also simply be a matter of a loose connector on the sensor  where it connects to the motherboard inside the device but to check it you'll need to open the bottom cover.  Unless you're suitably skilled and tooled I'd recommend taking it to an Apple-certified or skilled repair centre to check.

Have you tried setting sleep mode from the Apple icon in the upper left corner? Just to check whether the sleep mode itself is working (every time).

Ok update.
Spoke to Apple support,  apparently many users are reporting this problem that the laptop will not sleep when you close the lid.  They have recommended manually selecting sleep before closing the lid.  This is an apple hardware issue that they do not yet have a solution for.

So my suggestion was working ;)

Any Apple reference / link about the subject?

To help you put the device to sleep easily .. you could create an AppleScript and save it as an Automator script application and put it in the Dock .. you'd simply double-click the app and it'd put the device to sleep.
http://hints.macworld.com/article.php?story=2005080905050737

You can also use the HOT CORNER feature in the screensaver settings to put the display to sleep

Gerwin - it's not a solution as the system does not go to sleep automatically when the lid is closed.  

Scripts are not the way I want to go either.  I would like my laptop to work correctly and this doesn't seem to be a setting issue that I first thought but a bug that Apple has identified across their MB range.  

Went through lots of possible solutions inc setting up new profile, new partition and new OS installation but none of that made any difference unfortunately.  

Thx for the suggestions.

My suggestion about the SCRIPT was purely to help you perform the manual SLEEP until Apple address the hardware issue for you

But I can just select sleep from the menu and shut the lid  rather than write a script?

>> But I can just select sleep from the menu and shut the lid  rather than write a script?
Yes you can. Or press the power button for about 1.5 seconds.

You don't have any external devices like display, drives or others connected, right?

> Apple hardware issue

have you tried Eoin's suggestion to reset PRAM and SMC.

per my experience (i had the same issue before), it can be fixed this way.

Yes I spent a lengthy tel session with Apple resetting the SMC and various other things.  
As mentioned, Apple has identified an issue with their laptops,