Used stamps exchanges.

Some of my letters are not registered. There may be different reasons, but I feel uncomfortable asking the correspondent to register. What are the ways to help - auto-registration or something else?

After 90 days, you have the option to mark it as "Mailed". You can find the list of your current exchanges in the first page after you login. Your partner will also have the option to mark it as "Not received".
If you mark it as "Mailed" and your partner takes no action, the ID will be registered automatically after another 30 days.

you are right, myself I sent twice stamps to the same person, but the letter was not registered. really I feel like you - upset.