IG scheduling mystery

Hi all,

I'm an IG SMM with several clients. I can schedule all but ONE client's posts directly via Instagram. 

The odd thing is that this client can schedule with the same account. But when I am logged into their account I cannot. I tried on an additional device and still no luck.

They are a business account. Not a creator. Not a personal. 

I am baffled. Can anyone help?

Thx,

Zoe