API query not retrieving all records

@operator

I have an API call to retrieve all of the invoices that have been added/modified within the past week. However, not all of the invoices are returned when I make this call. Here is how I have built the transaction search:

// Create the search criteria   var tSearch = new TransactionSearchBasic {   lastModifiedDate = new SearchDateField   {     operatorSpecified = true,     @operator = SearchDateFieldOperator.within,       searchValueSpecified = true,     searchValue = LastModified,       searchValue2Specified = true,     searchValue2 = EndDate   },     recordType = new SearchStringField   {     operatorSpecified = true,     @operator = SearchStringFieldOperator.contains,     searchValue = TransactionType   } };   with LastModified set to the start of the seven-day period and EndDate set to the end. The TransactionType is set to "invoice". Is there anything else that I am missing?