There is several reason why your payment page is not showing:
There is no payment method in your store.
You have activated bypass payment page settings
One of your product setting's is "Display as estimated price"
Your customer place an order with Total order amount = $0
If you wish to show your payment page, the solutions will be:
Add at least 1 payment method - (Adding payment method)
De-activate bypass payment page settings (from Settings > Checkout)
Find all products that have "Display as estimated price" and de-activate this setting.
Have any order total amount bigger than $0