I have exactly the same problem... In my app, which is hosted by a page tab, the "Like" buttons for each item apper correctly and the "data-href" attribute for each "like" button has the same format as yours, but EVERY like button points to the Facebook page which hosts the app, not to the individual item... so, for every item in my gallery, the "like" information under it is exactly the same (number of likes for the page hosting the app) and if I "dislike" any item, I "dislike" the entire page.
I have tried the same format but the parameter NOT on a facebook page... then it works... I don t see a difference but:
https://www.facebook.com/pages/xxxxxx/xxxxxxxxx?sk=app_xxxxxxx&app_data=abc (DOESN T WORK, for each item which have app_data different)
https://www.mytestweb.com/test.aspx?sk=app_xxxxxxx&app_data=abc (WORKS for each item which have app_data different)