Hi Nisha,
This is because you are not associating the data. On the creation of Bank Details you need to set the assocation to contact details. In your domain model I would first create the Bank Details, and then change the contact details object (with commit) to set the association to the Bank Details to have the same functionalities.
On the button “Previous” you can then retrieve over assocation from bank details the contact details and show the first form with those details.
Please be aware that in the current flow on the next button, you will create a new bank account (again) so all filled values will be gone.