The first line of a plain-text file reads:
# -*- eval: (auto-revert-tail-mode 1); -*-
Yet Emacs does not enable
auto-revert-tail-mode after opening this file. What is wrong?
Take a look at the variables
enable-local-variables. If you've changed one or both of these from their default values it could explain the behavior you are seeing.
For example if
enable-local-eval is nil then that eval line in your file would be ignored.