Adobe Illustrator Guides Not Positioning Where I Want Them

[radderfire]

Hi Guys

I am new to Adobe Illustrator CC. I am trying to drag a horizontal guide to the edge of something, but when I release the mouse button, the guide is not dropped where I left it, but a small distance away. Why won't it just stay where I left it? What could be causing this?

Thanks

 

[radderfire]

OK I think I've worked it out, I was trying to position it at fractions of a pixel level, and must go to the nearest pixel.