missing parts - but they are there...

I'm having an issue with my assembly. All was ok until I changed a part (which wasn't actually related to the part I am now having issue with)

In this image, you can see I am pointing to 2 parts in the local folder. One is a washer and the other is a Bolt. They have been in the assembly for around a week. Yesterday, I changed a part that does not share any common relationship with these.

They are both in the same folder and both in my assembly but, SW is showing an issue with one of the parts. When i highlight over, I get a message:

"Cannot locate the file for the component to complete the operation"

When I right click and select Open, it opens the part just fine. (I was hoping for the browse dialog)

When I right click and select Resolve, nothing happens.

I have tried replacing the component with a copy version but it makes no difference.

Its like its there but not there......

I cant seem to fix it and seeing as I have about 500 instances of the part in the assembly (Its a M5 Clinch nut inside various sub assemblies), I am reluctant to replace them all and re constrain them. I shouldn't have to, they should work.

My File Locations is set up to look at the folder through Referenced Documents

any ideas on how I can fix this?

Cheers

Nacho