Please check your WF. There are possibilities that can occur here,
1. check the "Handler Advance" of your WF in the last step. If the advance handler is not handled in the code, then we need to select 'Handler Advance' checkbox to proceed to next step. You need to check the "Handler advance" value in the Process tab of the Process step. If the checkbox is ticked then the workflow will automatically move to the next step else you need to handle this programmatically in your process implementation.
2. If you are using a OR step do have one Default route.
3. Check if there are any errors in logs while executing the last step. Resolve the error if any to avoid incomplete WFs.