Earlier this year, TripAdvisor changed their site to a secure one (using https:, not http:) This means that Google Analytics can no longer track referrals from TripAdvisor without some “tweaking”.
If you have noticed a drop in your TripAdvisor referrals in Anlaytics, this is probably why.
There is a work around for people with paid TripAdvisor business listings (eg accommodation sites) but not for other listings at this stage (like activities or free accommodation listings).
The work-around involves changing your website link to a custom Google tracking url.
Information on this change is at https://www.tripadvisor.com/TripAdvisorInsights/n2692/faq-recent-changes-tripadvisor-site-security
We can assist you to do this if need be.