Download button missing for non-admins when multiple assets are selected
In AEM6.1 SP1 or SP2, when you select multiple assets in the /assets.html Touch UI interface, download option is missing in Card and List Views, however, it shows in Column View.
It is a product bug fixed in AEM6.1 SP2-CFP3.
Apply AEM 6.1 Service Pack 2 and Cumulative Fix Pack 3 or later from here:
Before applying a Service Pack or Cumulative Fix Pack to production environments, test it thoroughly in lower environments.