How can we apply a credit to an invoice with multiple installments?

I am trying to apply a credit memo on an invoice with multiple installments. However the credit memo is getting applied only on the first installment and leaving an open balance on the invoice and credit memo. I do not see any errors printed as well in the output. If i unapply and re-apply using the same API, the credit gets completely applied on all installments of the invoice. Code snippet is as below:

Please advise as to what is incorrect and fails first time with partial application?

declare:

CURSOR c_inv_trx (
p_cust_trx_id NUMBER
) IS
SELECT