This one solved same problem for me.

1. Press Win + R and type "Regedit" without the quotes.

2. Navigate to "Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\WindowsSelfHost" and click Applicability.

3. If you are anything like what happened to me then you will not find an entry called "EnablePreviewBuilds". So you need to add it. This is done by right clicking the blank space, hover over "New" and click "DWORD (32-Bit) Value"

Name it "EnablePreviewBuilds" and press enter. (CASE SENSITIVE!)

4. Now you need to right click on your new entry and click "Modify". Make sure your Base is Hexadecimal and change the Value data to 2. Press enter and exit regedit.

5. Now click on the windows logo, click the Settings cog/gear, and navigate to "Update & Security > Windows insider Program. Link your windows insider account (Make sure you have one) and the box should have changed from "Take control" to "Fix me". Click on "Fix me" and after it has finished restart your computer.

6. After restarting go back into your settings and back into the Windows insider Program tab and switch the Insider level to what you want. In my case I swapped it to "Fast". Voila! Your done.

*It may take some time before you get your new build in the fast or slow level. Give it a few hours or even a day and they should start coming through.* Hope I helped anyone who ran into the tread like me!