Sounds to me like the report's query is the problem. You probably have equal joins between fields of two or more tables. Try an outer join (left or right, depends on your table layout in design view). Research outer joins if you don't know what I mean.
Your criteria may also be the issue, especially if you are only using one table.
I agree with ranman256 - why bother with the combobox if you want all records - unless maybe you are running different reports from the same form and using the combo box for more than one of them. In that case, I would use the click event for the specific button to determine which report was asked for and set its recordsource to a different query in the report open event. Note that you do not need multiple versions of the same report to get different results. You just assign the appropriate recordsource on report opening to the same report.