Hi all,
Just a little FYI in case anyone runs into this problem - namely Xcode 11 was not updating despite the Mac App Store showing an update available.
For several days I would click "Update", and the progress wheel would start to spin but never progressed to actually downloading anything.
On a hunch this morning I re-enabled SIP (I had turned it off to for my endpoint development work), unloaded my daemons, and the Xcode update downloaded and installed just fine.
This is a single data point, so it may just be coincidence, but if you've disabled SIP and find Xcode won't update, give this a try.