When you are entering the bank payment voucher, if you enter the Check No. on the Document no. field, then the same will be available on the Outbox Transaction. I assume you are using IN loc. version, and you are entering the Check No. on the Check No. field on the Bank Payment Voucher Screen (which is IN loc. functionality) and the same is not available on the Outbox Transaction. So, you can ask your support partner to help you to make Check no. available on the Outbox Transactions, or instead use the Document No. field on the Bank Payment voucher.
cheer !
RLH