Hi Siva,
1. Please make sure that agent determination is correct for User Decision step. Please check in technical log to whom user decision step is going for execution. Also, whoever the agent is, please make sure that you have concatenated 'US' with user id. For example, if SIVA is the agent, then you should assing USSIVA as the agent for user decision step.
2. If agent determination is correct for user decision, and still no workitem is going into the inbox then please make sure that you have made the user decision task as general task.
3. If both above points are correct, and still user decision is not coming up in workflow log, then please check if the checkbox 'Not in Workflow Log' is checked for user decision step.
Regards,
Richa