Is it actually a subform that is losing the information?
If so, open the subform's table directly, and you will probably find that
the entries are there, but the foreign key field is Null. That means they do
not match any record in the main form, so - even though they are there in
the table - they never show up in the subform again. To prevent that form
happening, set the foreign key field's Required proeperty to Yes (in table
design).
If is is not a subform, what is the RecordSource for your form? A table? A
query? If query, post the SQL of the query.