Advanced Higher (Adh) SQA Biology Assignment

I was needing some help with my assignment, so I am the protocol in the link below, but my experiment isn’t fully working, when I use the colorimeter there is no change in absorbance over the 5 hours of which I do. I was also wondering if anybody could help with suggestions on what to write about in the Underlying Biology?

https://qualifications.pearson.com/content/dam/pdf/A%20Level/biology-b/2015/teaching-and-learning-materials/AS-and-A-level-Biology-B-Core-Practical-12-Bacterial-Growth-%28Student,-Teacher,-Technician-Worksheets%29.pdf