Custom Classes not appearing in Storyboard

My project is using a storyboard that has been working great so far. However, just yesterday something very strange happened - here are the details:

Any new IBOutlets or IBActions I create aren't listed under "Referencing Outlets" in the Connections inspector.
If I remove any of my existing referencing connections in the storyboard - they disappear completely from the list and I can't re-connect them.
If I try to set the custom class under the identity inspector, I can't see any of my classes being pre-populated or appearing in the dropdown list.

I have tried cleaning, cleaning derived data, quitting XCode and restarting my computer but no luck! This problem hasn't affected any other projects though.

If anyone has any thoughts on how this could be resolved that would be amazing!

Kind regards, Jeremy

Solution

I was able to fix the problem by restarting my computer again!

I think that you must need to clean your build folder and then restart.

Thanks guys for your comments!