Importing Macros doesn't work
I think you need to reference a macro name inside of your _common/_helpers file.

For example:

{{ helpers.input('username') }}


That would output a macro called input:

{% macro input(name, value, type = "text", size = 20) %}
    &lt;input type="{{ type }}" name="{{ name }}" value="{{ value|e }}" size="{{ size }}" /&gt;
{% endmacro %}