Make sure to do a Windows Defender scan then try again.
My 3rd laptop has finally quit doing that and after the 9th restart started the downloads.
EDIT: is now at 20%. into preparing to install.
Just keep after it.
My first one only took 5 restarts and the second only 4.
1 3/4 hours to restart.
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You can try forcing several restarts, It has been reported that it could help.
I would suggest to reset WinRM service via PowerShell - see my addendum at the end of this blog post: http://borncity.com/win/2017/01/28/windows-10-build-15019-insider-preview-released/
Fix: Windows 10 Build 15019 Insider Preview hangs with "Initialising updates"
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2 of my laptops are nearly the same and took different hard reboots to finally get the download started properly.
Just keep trying.
this thread helped me. "pc build 15019" windows update issues"
I let the computer sit all night with the same issue (no download}, then I saw this thread and had the new build downloaded in a few minutes.
This is the work around link from Jason@Microsoft:
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