Mac – Installing Xcode from App Store doesn’t seem to work

I don't know if this is your problem but look and see if you have any hidden purchases on your account. The way to check is:

1. Launch the app store & Select Store > View My Account and sign in.

2. Look for a section headed iTunes in the Cloud, with a Hidden Purchases entry, and click View Hidden Purchases.

3. Click Unhide for those purchases you wish to appear in your App Store Purchases window, then click Done.

Let me know if this helps.

Maybe this thread on Apple Support Communities is what you're looking for.

It sounds like it is, indeed, a cache problem. From the thread above, it'd seem you'd need to find the cache in the /private directory. I'll include specific instructions, inspired by the thread:

1. Open Terminal (/Applications/Utilities/Terminal.app)
2. Type in this command: sudo find /private/var -iname "com.apple.appstore" and press Enter
3. For each of of the found directories run rm -r <dir path>/* (For the brave: sudo find /private/var -iname "com.apple.appstore" | xargs sudo rm -r)