Linked properties not updating between files

Hey all. I have created a blend file, with has custom properties on the object. All good. I imported this into another blend file and animated those properties. Again, all good. I did this via proxies.

But then I went back to the original file, and added an extra custom property. I actual split the first one into 2, so I had more control over the item itself.

So, to begin with , two engines glow on one parameter. I split this (in the new version) to 2 engine controls - so I can control the glow of each engine.

In the main blend file which has the linked objects, the GLOW works on ONE engine as expected, but the new LINKED property for the second engine is NOT showing up and I cannot work out how to get it in.

I have found out that if I delete it, and readd it all is good, but I have shed loads of animation on the linked object which ideally I dont want to lose.

Also, going forward, I really don’t want to delete and re-add anything everytime I change a base object.

Am I missing something? I tried the relink from file, refresh and everything obvious to click on.

Or can I delete an object and keep the animation, re-add and apply the animation?

Many thanks,