We are using the reporting API for multiple clients to get the data from Adobe Analytics. There is a big problem where for some clients we keep getting "report_not_ready" error after waiting for around 10 minutes. Also, for the same client we make the same requests everyday and on some days it may return the data.
This wholly depends on the report complexity. If it's a complex API request using multiple breakdowns, metrics, and segments for an extended date range, it is very possible for the request to take longer than 10 minutes.
Your best bet is going to be to either wait a bit longer, or simplify the request so it returns faster. You might also have luck with the v2.0 APIs, which were released last month.