You can force the update, but in a somewhat manual sense. I have two personal windows 10 installs that are running the insider 'preview' edition. Both refused to update, most likely due to the unnatural (per MS) configurations that I keep them in - mostly simple stuff like non default directory locations.
There are two similar methods to update to an insider build if the automatic method is not working - two that I've used, in any event:
Method 1:
Download the Insider ISO and do a manual upgrade. A manually triggered update is no different than what Windows Update will do; each new insider release is treated as a full OS upgrade. Go to
https://www.microsoft.com/en-us/software-download/windowsinsiderpreviewadvanced
and download the appropriate ISO and install it. The official ISOs are not generated for each release; at this point in time the ISOs contain release 14295. However, and this is key, once you have installed a fresh insider build, anecdotal and personal experience is that the automated updates just start working.
Method 2:
To update to the latest build for your system, you can leverage the installer that is downloaded automatically by Windows Update - even when you, the user, is informed that no updates are available. On Win10, this is nearly always located at C:\$WINDOWS.~BT
You will need to have enabled viewing "protected operating system files" in order to see and browse this directory. It's very likely that the full installer for the latest insider build is already on your computer, in a file titled install.esd that is typically ~3GB in size.
Windows Update downloads the installer as install.esd, in the Electronic Software Delivery format; which is an encrypted installer package; equivalent to a .wim + encryption. It is trivial to turn this file into an ISO, load it on a usb flash drive, and install it directly. There are numerous tutorials on it, so rather than reinvent the wheel, here is a good one:
http://www.windowscentral.com/how-create-windows-10-iso-file-using-installesd-image
This link also discusses how to trigger Windows Update to at least download the latest install.esd so that you can install it.
Summary
The bottom line with both approaches is that there is a state that your windows install can get into in that prevents insider updates from being installed. Windows Update and related log entries provide no substantive information on what the issue(s) is/are, and you are simply told via the Win Update GUI that there are no updates available. If you are stuck on an earlier release and want to get to the current; give this method a try. I hadn't paid attention to the lack of updates I was receiving until I wanted to check out the BASH shell beta that was introduced in 316. It turned out that both installations were stuck on the August release; that drove me to look into this.
Doing a manual update seems to reset this state such that future releases are installed promptly and without issue. It's a bit vague, but without any information on why the updates are being presented to their beta testers as 'not available', it's an enormous time sink to speculate on a more sophisticated / less brute force method to override it, and so far at least, no one has come up with a working solution that is more evolved than keep clicking the check for updates button and hope for the best.
One last caveat - you can do this - method 1 at least - even if you are not currently signed up as an 'insider previewer', however before doing the upgrade you should sign up for the insider preview with the MS account you use to login to your win 10 machines. Both are required to participate in the program per MS (specifically you cannot solely use a local login account). It is currently Undefined BehaviorTM what happens if a non insider preview account is running an insider build.
