Well, I find this feature extremely useful and have begun to use it.

Although,I've noticed that it doesn't work with one of my contacts.

As in, I right click on that certain contact and the options of:

Display on Desktop
Events Notifications
Open Chat Log
Contact Information

cannot be clicked.

Except for the last one, which is Copy E-mail to Clipboard.

Why does this happen? And only to that certain contact?

How do I  fix it?

This is a known limitation for contacts who have different email addresses set as their main address (you can see that ehen editing your contact's info).

There is an easy workaround though: click on the status icon left to your contact's name and in the little window that popup-ups, click on "Additional options", you'll be able to do anything you want from there .

And also there is a dedicated window for desktop contacts, you can open it from Plus! menu>Contacts on Desktop.

You can select the contacts who you want to see on your desktop as a whole, also you can set nicknames to use only in your small floating desktop contact windows. To set a nickname to a contact, double click on him/her.