CItation disambiguations

AMeganGee

I have been modifying a style to meet our internal requirements. Only some citations are using the a, b, disambiguation, the rest have the same citation even though the authors are different. It appears to be ignoring differences in later authors. I went back to the original style (Taylor and Francis - Council of Science Editors (author-date) and it has the same issue. Any suggestions please.
e.g.
Individual Citations
(Chen et al. 2021)
(Chen et al. 2021)
(Chen et al. 2021)
(Chen et al. 2021)
(Chen, Zhao, & Wang 2021a)
(Chen, Zhao, & Wang 2021b)
Bibliography
Chen S-C, Zhao F-H, Haenke S 2021. Authors-1st 2 same. Mitochondrial DNA B Resour 6(3): 875–877. DOI: 10.1080/23802359.2021.1886009
Chen S-C, Zhao F-H, Wang X-Q 2021a. AUTHORS-SAME A. Mitochondrial DNA B Resour 6(3): 875–877. DOI: 10.1080/23802359.2021.1886009
Chen S-C, Zhao F-H, Wang X-Q 2021b. AUTHORS-SAME B. Mitochondrial DNA B Resour 6(3): 875–877. DOI: 10.1080/23802359.2021.1886009
Chen S-C, Zhao F-H, Wang X-Q, Haenke S 2021. Authors-same 1st 3 same: 4th diff. Mitochondrial DNA B Resour 6(3): 875–877. DOI: 10.1080/23802359.2021.1886009
Chen S-C, Zhao F-H, Wang X-Q, Haenke S 2021. Authors-same 1st 3 same: 4th diff test. Mitochondrial DNA B Resour 6(3): 875–877. DOI: 10.1080/23802359.2021.1886009
Chen S-C, Zhao F-H, Wang X-Q, Haenke S, Jacot K 2021. Authors-same 1st 3 same 5th diff. Mitochondrial DNA B Resour 6(3): 875–877. DOI: 10.1080/23802359.2021.1886009