Did you receive any uplinks in the past hour? (Assuming the integration was enabled at the time the uplinks were received.)
If you’re using a Decoder in TTN Console, then beware that the Data Storage Integration silently fails for attributes with null values.
Indeed, that may surely help. The default is 1 hour.
Asides: note that a non-existing device ID will return 204 No Content too (if it does adhere to the naming rules). And note that the integration will store at most 7 days of data. There may also be some limit for the maximum number of results, but one should not run into that when adhering to the Fair Access Policy.