there is a process model were it passes a CDT as paramter to a sub process ,the

Please make sure you are don't have pass by reference selected while passing CDT's and from the process history tab for that particular instance you can trace when and where CDT value get nullified.

Please also make sure that output parameters of the sub process do not have same CDT configured. Output parameters of sub process are hidden when you configure sub process to run as a asynchronous process. It is essential to remove these parameters at design time to avoid data issues.

thanks but its not passed as a reference and its not set as output , any other ideas ?

Can you confirm that CDT is not nullified before it hits and comes out of the Sub Process? Also check the value of CDT variable in the Sub Process, if it is null that means somewhere in the parent process CDT stores null value. And if it is not null, then check if there are any other script tasks right after the Sub Process node just to eliminate the possibility of having an input / output field modifying CDT variable.

the CDT is passed as a parameter and used in the sub process but during the execution of the sub process the variable (CDT) is set to null in the parent process - no script tasks updating the CDT in the parent process and its not passed as output or as a reference

You can turn your focus to the way you are populating your CDT. If CDT is populated through a query rule or an expression rule, you can shift your focus to these areas for investigation.

You can try two things
1. Check the source process variable being used in parent process - sub process node whether it has any value or not while being passed.
2. In sub process you can check the target process variable which receives the same

You can trace from process monitor history tab whether your source pv and target pv are getting intialized or not