The alerts themselves are not stored in the tdc_contributing_object table. That is where the "contributing objects" (also known as contributing transactions) are stored for each scenario-fired event.
Record-level scenarios save the key of the record that caused the scenario to fire in that table. Aggregation scenarios save the transactions that make up the aggregation IF you enable that option. If you are running aggregation scenarios, verify that you have checked the box that says "Indicate which records caused the scenario to fire" (see screenshot below). If those conditions are met and you are not seeing records written to the tdc_contributing_object table, please reach out to tech support.