Why can’t I see the flame parameter?

There are two possible reasons:

1. Start of a new album

At the beginning of each new album, the flame parameter is temporarily disabled.
This is because users have not yet accumulated enough duplicates: as a result, almost all stickers would appear “rare”, even 1 or 2-star ones, making the parameter misleading.
It is automatically re-enabled once it becomes meaningful again, usually within about 14 days from the start of the album.

2. Sticker too common

When the feature is active, flames are shown only for stickers that reach a certain level of rarity.
Very common stickers in the community may not display any flame.