My brother and I had the same problem and found using an old minecraft trick( before there was a force update button ) solved the problem.
Going into the roaming folder and deleting the mod pack's folder you're trying to update forces it to update without a problem. I haven't found a cause for this to happen, but it is strange.
Just in case you don't know how to do this here is a little tutorial:
1. Open the start bar and search for %appdata%.
2. Open the folder called .techniclauncher
3. Locate the mod pack (or vanilla) that you wish to update
4. go into it and save any maps or texture packs ETC. that you want to keep
5. After saving the above (if needed) hit back and delete the modpacks folder
What this does is it basically makes it so if you never played the modpack before and downloads the most recent version.
If you need any other info just reply! :D