The carbon-13 resonance of quaternary carbon atoms is characteristically weak, due to the lack of nuclear Overhauser effect and the long relaxation time, and can be missed in weak samples, or samples that have not been run for a sufficiently long time.
ðə kˈɑːɹbən θˈɜːtiːn ɹˈɛzənəns ʌv kwətˈɜːnɚɹi kˈɑːɹbən ˈæɾəmz ɪz kˌæɹɪktɚɹˈɪstɪkli wˈiːk, dˈuː tə ðə lˈæk ʌv nˈuːklɪɹ ˌoʊvɚhˈɔːsɚɹ ɪfˈɛkt ænd ðə lˈɔŋ ɹᵻlɐksˈeɪʃən tˈaɪm, ænd kæn biː mˈɪst ɪn wˈiːk sˈæmpəlz, ɔːɹ sˈæmpəlz ðæt hɐvnˌɑːt bˌɪn ɹˈʌn fɚɹə səfˈɪʃəntli lˈɔŋ tˈaɪm.