Setting Form RecordSource property returns blank form when no data matches SQL query

[tina]

have you tried not changing the recordsource for either
form, but instead filtering the main form's data by month
using the Me.Filter, and Me.FilterOn properties?
if the main/subform objects are linked properly as
parent/child, then the subform will automatically filter
to present only the records related to the filtered
records in the main form.
if the filter results in no records, then no records will
show in either the main or sub form.