Some of my Domains are Missing
The domains that show up in your list of My Domains are those domains for which one or more of the contacts on those domains match the email address and password (credentials) you logged in with.
If one or more of the domains that you own are not showing up in the list My Domains, this is probably because the email addresses or passwords of the contacts (for the missing domains) do not match the credentals (email+password) you have logged in with.
You can use contacts with different passwords, but the same email address, to keep your domain names in different groups. Or you can use a single contact Id on all the domains if you wish to keep them all together.
NOTE: it is important that the Owner Contact record correctly reflects the owner of the domain. However, the domain's Admin Contact has the same priviledges, so the Admin Contact can be used if the person / organisations looking after the domain registrations is different from the one that owns the domain.
Solutions to missing domains
If you do not know the password of the contacts of the missing domain you may need to use the Password Reset option to reset it to a known value.
When you use this option a reset web-link will be e-mailed to the contact you specify.
Once you have recovered your missing domains, we STRONLY recommend that you review the number and use of contact records within your domains and reduce the number of contact records you use to the minimum possible.