Thanks.

does it just align slightly to one side or the other?

zoom in further to get the more detailed ruler marks.

Nope, it doesn't align at all.

can you see the guides at all ? ctrl+H turns the viewability on and off.

are your guides locked and not on the screen?

If the guides can't get to the centre, try using something visually to help you instead.

Open a new file that is half the width of your original file you are working on.
Create new layer. Fill it and then drag it into the original file.
Make sure Smart Guides are on.
When you drag the new filled half sized image to the left you will lock onto the edges.
Turn transparency down and you will get a visual guide of where the centre is.

Then try to align new guide and it will lock center?

Like this

Or it could be simply to turn off all SNAP TO

Menu
View -> Snap to>

TURN IT ALL OFF.

Your guides are snapping onto the ruler segments rather than where you actually drag to?

Thanks kidclam.
The Snap was turned off.

Has this been resolved? I still don't really get why you can't move guide to the center.