Did you check for any active locks on the file? I had a similar problem for both ArcMap (Properties: Symbology) and in ArcCatalog for a layer that I had recently created. I had edited it using Python using an Update Cursor, and even though that should not have left a schema lock, once I closed the Python window, I was able to edit the Representations again. So I'd recommend checking for other places (ArcMap, Python) using the data set, or other users using the data set, and make sure that you are the only person actively working on the layer and ArcCatalog is the only location accessing it. This sometimes requires closing ArcMap documents that once had the layer open - sometimes it doesn't want to give up those schema locks!
Also "hence the fc is not being edited" is not true to my limited knowledge of representations. My understanding is that Representations are stored WITHIN the feature class attribute table, so edits to the representations are edits to the Feature Class.
I realize this is probably too late to be of use to you, but this was the top search result when I had a similar problem, so I figured I'd post what worked for me.
