For some surveys the participants need a specific profile (age, education, hobbies...). In other words, it is perfectly possible that you are invited less frequently because we are looking for people with a profile which is different from yours.
Check the e-mail address you entered when you joined the panel. People have been known to make typos in their subscription or to change their e-mail address without modifying their profile in the panel. You can always consult your e-mail address and other personal particulars here.
To avoid that our e-mails end up in your spam, we advise you to add email@example.com to your contact list or address book.