Since May 8, 2018, poe.trade only displays sellers online if they are playing in the league that you are searching in.
This only applies to people with premium stash tabs. Because the new online API only supports premium stash tabs, if you are still using Acquisition or another software to manually list items, you will appear online everywhere.
For people with premium stash tabs who'd rather appear online in all leagues, the setting can be changed by going to your personal control.poe.trade URL and clicking on "be shown online everywhere".
To retrieve your personal control.poe.trade URL all you need to do is message poexyzis on pathofexile.com with any subject and message body. Just click this link, enter any subject/content and press send. Your personal control.poe.trade link should arrive in your inbox within a minute.