Trigger on Case is not firing after Submitting Case from WebToCase Form

Hi there ,

I am having a simple WebToCase form from which I am submitting Case to my SF org . The case is successfully creating in my org /sandbox.

But I want to do some manipulation on this case record coming from form. So I have written trigger to do the same .

Problem :
Trigger is not getting fired after submitting case . I have tried all the events before and after on trigger . But not able to trace this case record in the trigger .
I have some assignment rules on it . I tried with activating /deactivating it . I have checked the Api version of the class/trigger .

Any workaround on this issue ?

I have implmented it in a developer edition and its successfully firing the trigger . But in sandbox its not working :(

Thanks ,
Ajay