Comment 5 for bug 964146

Short story:

I think unity-lens-applications should Depend (not Recommend or Suggest) zeitgeist OR zeitgeist-core for installation. On some system the zeitgeist daemon may not be installed, thus causing the lens to malfunction.

Long story:

I have configured APT to prevent installation of soft-dependencies (Recommended and Suggested dependencies) like this:

APT::Install-Recommends "0";
APT::Install-Suggests "0";
Apt::AutoRemove::RecommendsImportant "0";
Apt::AutoRemove::SuggestsImportant "0";

Now, unity-lens-applications depends on libzeitgeist which in turn Recommends zeitgeist OR zeitgeist-core. Since it is a Recommended package it is not installed on my system, thus the zeitgeist daemon is not started.

I had to manually install at least zeitgeist-core, then restart unity-applications-daemon to have it spawn the zeitgeist daemon. After that, the lens worked as before.