key generation issue - using same source & target

The business requirement is to generate surrogate key on unique field, other than primary key.

I have used source & target to generate primary key based surrogate key. No issue, working fine.

But in new data flow, I use same source and target to generate surrogate key for unique field using key generation. The job is running successful but key column in NULL.

Is there a way to solve the issue?

Thanks..