User cannot see retainer history on an invoice.
Typically, BQE CORE allows you to see the account summary as well as the retainer history on the invoices. If you are unable to see the retainer information, it is possibly because the retainer is applied after creating an invoice via the Use Retainer payment method. You can enter and apply the retainer while creating the invoices, or in the Subtotals view of the Time & Expenses tab in the detailed view of the invoice while it is still a draft.