default lead and archives

Hello,

on post’s that have an certain age (or maybe it’s after number of n posts), the default lead section (title) is not displayed anymore when it is viewed.
While I figured out, how I can disable the default lead, I just don’t find the reason why it’s not displayed when you, for example, display the archives for let’s say June and click on one of the posts.

Please help me with that, also I would find it a quite useful option to enable/disable lead section theme-wide for defaults like it’s possible with other options.

thank you!