It should show all open"applicable" transactions of the customer. In your case, are you missing ALL THE CREDIT MEMO or is just a single 1 test?

@Andrea Rivetti while creating a new customer payment it shows all credits, but when i edit payment its not showing me other ccredits where i need select few more on editing

Are you starting from a specific Invoice or from the customer alone? Is it possible that the customer does not have any other open invoice and so you can create the payment as a standalone (like a deposit) payment? (you can't choose what to apply because you don't have any open credit)