I think one of the recent updates changed the key binding for this from the Shift key to the Z key. The Unity3D engine that we use has problems figuring out that a key has been released if the game loses focus, so things like shift+tab, used to control the Steam overlay, cause shift to get stuck, which was causing a lot of pain for a lot of people. Unfortunately, we have no control over this bug, and the Unity team has yet to provide a fix. So, the solution was to change the default to Z, which would address the problem well enough. That would explain why it stopped working for you after an update.
You can change this back to shift, if you'd like, with the Key setting in Settings -> Change Key Bindings -> Make LMB = RMB.
If it turns out this doesn't fix it, you could try removing or renaming your settings.dat and settings.bak files in Tidalis/RuntimeData. These contain all your various settings, achievement information, and your license key information (but not adventure mode saves). If the feature works after that, then it is some other setting that somehow changed, and if not, I suspect it could be something else related to an update somehow. If removing the files didn't fix it, you can restore your old settings.dat and settings.bak files to get all your info back in the game.