Often times the Gemara will attempt to understand the words of a Tana or Amora, and goes through a few possibilities until understanding the exact Derasha used by the Tana/Amora. For example, in the case of Rav Huna, the Gemara originally thought he learned a Kal v'Chomer (even though it says "Amar Rav Huna" it is probable that he did not actually say more than "Chupah Koneh," and the Gemara filled in "mi'Kal v'Chomer etc.", trying to understand his source). However, after the Gemara rejected that as incorrect, the Gemara switches gears and understands that Rav Huna's source is a Meh Matzinu, which soon turns into a Tzad ha'Shaveh (as do many Meh Matzinu's that are challenged).

Amora'im can certainly expect to understand the logical teachings of the Tana'im, although the Gemara will not usually say that an Amora's question uproots the statement of a Tana. This is for many reasons, primarily because the Amora'im were trying to understand the Tana'im, and usually found an explanation for their statements. Even if they did have a good question, I do not recall the Gemara ever saying "Tiyuvta" in such a case, as the Tana clearly had his reason which we are merely not privy to understand (see Nidah 31b where Rav Yosef asks a question on a statement of Rebbi Shimon Bar Yochai and the Gemara's discussion abruptly ends without an answer, but without saying Tiyuvta).

All the best,

Yaakov Montrose