My flair only shows Physics SE and Math SE. I want it to make it show world building SE, but I don't know how to make it.
I have more than 200 reputation there, and have not hidden it.
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We aggregate a bunch of information about users in a single table that is then used by the flair generator. It looks like something went wrong with the aggregation in your case. Specifically, we had a record for your Worldbuilding user in that table, but it wasn't correctly attached to your network account.
I fixed that up, so next time the flair regenerates, you should see Worldbuilding on it. As a side benefit, your network profile on stackexchange.com now accurately reflects your badge and question/answer counts.