If your model has the multiple columns line as the DETAIL template and the Comments Template set up as an APPEND template then you should get what you want. From what you wrote I am wondering it you have those the other way around.
Your Comments field in its[/i] Template needs to be a multi-row field definition so that it will continue for as many rows as there are for each comment AND it needs to have a way to identify when it should START such that if no comments exist then the field is not populated. Both the start and the end attributes of the field can be set up using its Advanced Properties.
The first thing to do is check which template is set to DETAIL and which to APPEND as mentioned above. (I have assumed you have both templates using the exact same trap.)