Hi Lauren and welcome to the forum.
I assume you are using a filter to reduce the number of rows displayed in the table.
If so is your troublesome summary set to use the default filter (which would be the filter currently applied to the table) or does it have its own filter or No filter? (I would assume the Default filter).
When you save the model and then re-open everything I assume you are saving with the filter applied and it therefore becomes the default filter when you re-open the report and model.