It's a known issue with a conflict for newer OS versions!!
I cannot post the link so here are the instructions.
Search for "I have a supported device for Windows 10 Technical Preview for phones, but I don?t see any updates available for install" on Windows Insider Program
If you follow this procedure you will get access, it worked fine for me on a Lumia 635
If you selected ?Insider Fast?, and you still aren?t offered the upgrade, on some devices there is a software conflict that will prevent a successful update to Windows 10 Technical Preview for phones. When this conflict is detected, no software updates will be shown for installation.
To find out if your device has this conflict, please check the OS version installed.
1.Navigate to Settings > About > (more info)
2.View the OS version. If your device shows 8.10.14226.359 or greater, your device has this conflict. Devices with other OS versions do not.
Should your device have the conflict, please follow these steps to update to Technical Preview:
1.Double-check to make sure your phone is on the list of supported devices for Technical Preview. If your device is not supported, you cannot install Technical Preview at this time.
2.Using the Windows Phone Recovery Tool, begin the process to re-flash your device.
◦This will recover the device to a software build that does not have the conflict.
3.After the re-flash is complete, DO NOT CONNECT TO WI-FI. This step is critical to prevent the device from automatically taking a background update to the OS version that contains the conflict.
4.Once you reach the Start screen, go to Settings > Phone update and uncheck the box for ?Automatically download updates if my data settings allow it?.
5.Perform a power cycle on the device, turning the device off and then back on.
6.Turn Wi-Fi on and download the Windows Insider application to the device
7. Launch the Windows Insider application and select the desired Insider state (Slow or Fast).
8.Go to Settings > Phone update and tap Check for updates.
9.Accept the update and follow the on-screen directions to complete the process for updating to the Technical Preview.