How can I free an RDS drill bit from my hammer drill?

I have a Ryobi mains-powered hamer drill that has an iron hold on an 8mm masonry RDS drill bit. It's the push-and-click chuck. This happens all the time, but the other drill bits go in and out with no problem, only the 8mm bit, which is really the only one I ever have cause to use. I've removed the from section of the drill to make it easier to handle but there's nothing accessible from inside apart from a ton of grease. Apart from drowning it in WD40, does anyone have any ideas on what to do, and more importantly, how to prevent it happening again?! Many thanks.