Same reference formats in two different ways in the same document

I have a reference which is used 16 times in one document, however sometimes the citation is formatted correctly:

(L. Archer, J. Moote, et al., 2016b)

While other times the exact same reference is formatted with the authors first names:

(Louise Archer, Julie Moote et al., 2016b).

As you can see, there are multiple authors for this reference. I have also cited a number of other references by the first author, some of which have multiple (different) authors and it is worth noting that I am also citing references by a different author, Margaret Archer. 

I have checked the reference file for consistency of input, I hace checked and unchecked various boxes in the output style related to ambiguous citations. I have even copied the correct citation and pasted next to the incorrect one but is then gets reformatted incorrectly.