Sounds (roughly speaking) like an incorrectly structured report. I have to take a shot in the dark on this because you offered so little information.
I would first design a query that was the JOIN of the main form's data and the sub-form's data. The query would APPEAR to have lots of repeated data for each entity in the main form, repeated once for each line item.
Then I would build a report based on the JOIN query, using the report's Group Heading (and Footing, where appropriate) to congregate the repetitive data. Last, I would make the line items the detail records of the report.
EDIT: And of course, if you are actually printing a form rather than a report (as GPGeorge suggests), that might be a mistake anyway. Forms are oriented towards showing information piecemeal. Reports tend to be more wholesale. Depending on your sub-form's layout, it is possible that Access is building a series of snapshots, one for each line item, because of the way you laid out the form.