Inconsistent online pick up times

[TWF19]

I have an issue with scheduled pick up times. I run a pizza food van and allow my customers to order online. They choose from the available pick up times that I have set up in the fulfilment tab. I set my prep time to 15 minutes, and only 1 order can be picked up every 15 minutes. I have two days I’m doing business this week, Friday and Saturday. Saturday is currently showing pick up slots available every 15 minutes as set up, however Friday is showing pick up slots every 5 minutes. What would be the reason for this? It’s very frustrating as now I am receiving orders that I will not be able to fill on time and there is no way I can seem to change this. Please help, urgently.