Why does my query run sometimes in parallel and most of the time not?

Hi

I have a query which runs in my QA and PROD database.

Amount of data is the same. And execution plan changes quite often, but is the same on both environments.

In my QA database the query most of the times run in parallel, but in production it runs in serial.

How is that possible?

What am I doing wrong? There are not very much other processes running at the same time (based on what the application manager told me and AWR is showing), so I don't expect it to be downgraded due to lack of parallel processes, but as it sometimes runs in parallel you should say that's the reason. I don't want to run a 10053 trace yet as it runs in the middle of the night and this generates of course quite amount trace-files. Can someone help me or push me in the right direction? Can we force to run it in