Why isn't the T in "relative" flapped?

One very common phenomenon in north-American English is T flapping when the T comes between two vowels (or semi-vowels, like the R sound) on an unstressed syllable.

This "rule" is almost mathematical, I didn't hear any T pronounced as /t/ in this environment until I heard north-Americans say the word relative(s).

I went to youglish which gives you youtube videos according to some word and dialect, and so far all of the speakers from America say /ɹɛlətɪv/ with an un-aspirated /t/ sound.

My questions are:

• why isn't the T flapped?
• does /ɹɛləɾɪv/ sound weird?
• are there other words which apply to the T flapping rule but the T isn't flapped?

Thanks!

After @Araucaria gave his answer, I decided to try more speakers, and here I found some:

https://youtu.be/AchISJUKfH4?t=706
https://youtu.be/bS45Ml30h9g?t=279
https://youtu.be/tbuLHSzBzZk?t=2175

In conclusion, most of the Americans don't flap their T in "relative", they pronounce it either aspirated or non aspirated. A very small minority do however, flap their T there.

I guess all of the answers given here are correct. Americans who don't flap their T in "relative" pronounce this syllable with a secondary stress, while American who do flap their T there obviously don't. I guess the amount of "stress" put there depends on the regional accent of the speaker.

I also guess that according to the "General American" standard, the T shouldn't be flapped there, as dictionaries do mark the tiv syllable as having a secondary stress.

Short answer:

There's no doubt at all that the /t/ in relative may be flapped in standard American English. Indeed many dictionaries include a flapped /t/ in their transcriptions and give audio examples with flapped /t/'s too.

Here for example is the transcription from Cambridge Dictionaries:

relative

noun [ C ] UK ​ /ˈrel.ə.tɪv/ US ​ /ˈrel.ə.t̬ɪv/

That little upside-down hat under the 't' represents a flapped /t/. The audio there also uses a flapped /t/.

And here is the transcription from Oxford Dictionaries clearly indicating a flap for a /t/, where their convention is to represent flapped-/t/ with 'd':

relative /ˈrɛlədɪv/

In spite of the above, the /t/ in relative is only optionally flapped in this environment. Many speakers may not do so, and others my only use flapped /t/ in certain circumstances, for instance in connected speech as opposed to when giving a citation form. For why, see the full answer below.

Full answer:

Here is an excerpt from a paper by Alice Turk, Professor of Linguistic Phonetics at Edinburgh University, concerning the phonology of flaps in American English:

Here we are concerned with examples like (2) above, where a /t/ occurs between two unstressed vowels. Turk's example, provocative, has the same morphology as the Original Poster's word relative. In both instances the /t/ is intervocalic (occurs between two vowels), and occurs between unstressed vowels.

As described by Alice Turk, the flapping of /t/ in this environment is optional. This is well-attested in the comments here, where several respondents point out either that this /t/ is flapped, or that it can be.

In short then, [ɹɛləɾɪv] doesn't sound odd at all. Whether an individual happens to flap a /t/ in this sort of environment will depend on many factors: the personal habits and predilections of the speaker, whether they are saying the word alone or in a sentence, the speed at which they are talking, and so on and so forth.

^Note:

^{No printed dictionaries give a transcription of relative with a secondary stress.}

^{It has been suggested elsewhere here that the last syllable of relative has secondary stress. It doesn't, at least not in the normal meaning of the term.}

^{In words with secondary stress, it's possible to have a rhythmic stress on both stressed syllables:}

^{- 'abso-'lutely fan-'tastic.}

^{In the utterance above we can have a full stress on each of the three bolded syllables, including both the first and third syllable of absolutely. We cannot do this kind of thing with the last syllable of relative}

- 'famous 'rela-'tive (stress on -tive, badly formed).

The word relative /ˈɹɛləˌtɪv/ often has weak secondary stress on the /ɪ/, which blocks flapping.

This is the same reason that the pronunciation /ˈɹɛlətəv/ sounds strange. People with the weak vowel merger generally replace /ɪ/ in unstressed vowels with /ə/. And if you live in the U.S., even if you don't have this merger, you are probably familiar enough with this merger that it doesn't sound strange. But /ˈɹɛlətəv/ and /ˈɹɛləɾɪv/ sound weird to me because the /ɪ/ has weak secondary stress—weak because it's a weaker stress than most things that are classified as secondary stress in dictionaries.