Why SignalTap II not working consistent?

I am using SignalTap II to debug design, but observing several strange behaviours of the instance. With the same instance where all the trigger condition are set "Don't Care", some times I could tap successfully with the instance status "Offloading acquired data", but many other times, the status is flicking between "Waiting for trigger" and "Acquiring post-trigger data". I wonder why the status can be "Waiting for trigger" as the trigger conditions are "Don't Care". And does the status mean that it needs more time to capture enough post-trigger data to fill the configured buffer? Here is how I configure the instance:

Sample depth: 512

RAM type: M4K/M9K

Storage qualifier: continuous

Trigger flow control: sequential

Trigger position: post-trigger position

It looks like I don't know how to use signaltap properly as it behaves not consistently many times. Anyone can help to explain and give me some recommendations for using this tool.

Thank you very much for your reading this thread and any comments are very appreciated.