"Do you still have a headache?"
Translation:Stále tě bolí hlava?
Okay, one hour later I finally got this sentence again. :)
"Stále vas bolí hlava?" was marked as incorrect and the app suggested "tě" instead, but with right accent in "vás" Duolingo marked my sentence as correct. It is just one of Duolingo random bugs I guess.
Děkuji za rychlou odpovědět. :)
In "Nevšiml jsem si tě.", the "tě" is also in the second position. In fact, all of "jsem" "si" and "tě" are. See https://forum.duolingo.com/comment/31466920
In the same link read about constant and inconstant clitics. It answers your question about all personal pronouns. Some are constant clitics, some are inconstant clitics and some are not clitics at all. For example, tebe is not a clitic at all. There is also more in our Tips and notes.