Solution not turning amber

Hello all.
I've bought MMS solution and HCL activator ready-made - exactly at the right quantities as set out in the guide.
However the mixing (activating) has only ever created a yellow colour. I did reach out to the seller who seemed surprised but issued a replacement set. However I'm still getting the yellow colour.
Does anyone have any thoughts, feedback, ideas about this?
I assume that I'm getting something from it but probably a lower dosage?
Many thanks, Zoe