The reason why some appointments aren't showing up in the Add Insurance Payment page is because the appointments you're looking for are set with the wrong Billing Type. You need these appointments to be set with the billing type Insurance but they're set up as Self-Pay.
To resolve this, you will need to go to the client's Billing Page. Once there, place your mouse cursor on the appointment fee for the appointments in question > click on the Edit Button > change billing type to Insurance.
Once you've done this, you will see that appointment(s) appear in the Add Insurance Payment page. If you need help adding an insurance payment, click HERE.