Does your Cesium map have a terrain? Could the data be under the terrain at 0 elevation? Have you checked the network section developer mode in your browser (F12) to see if .b3dm files are getting downloaded? I have found that converting to WGS84 gives best results in our maps, however, this could be due to configuration.

Also, if you add a background map to the data inspector and set the coordinate system of your data to LL84/WGS84 you should see your data in the right place in the 2D view (it will look really weird in 3D view but that's normal when in LL84).

If the coordinate system is showing 'Unknown' then the data are likely to be placed incorrectly. If they are 'Unknown' you need to set the coordinate system of your input geojson file

So i added a map box background and i now see the problem. The shapes are tiny and not visible when i am not zoom very far in. I tried changing the coordinate system but tat had no effect. Do i need to scale? or transform the lat/long?

Scratch that last comment i had to reload the view. So i now see the shaped on the map correctly, but i still can see them on cesium. I am going to try and play with the elevation. as Suggested abbove

I found the problem. When i tried the tileset in cesium sandcastle it worked. It has something to do with the cesium configuration in my app.