Amazon Ads and ROAS for a new product in stationaries

Hello, Please help. I started with £7 a day had a ROAS of £1.56. According to the website recommendations I increased my budget to £25 and my ROAS fell to £0.8-£1.2 max and I ended up with a big charge.

1) why did that happen? It’s like Amazon want to tell you it’ll payboff if you spend more on ads and then they “ruin” your algorithm. For the record I listed in December 2025.

2) I was also shocked to find out that Amazon are taking nearly 50% of my item. Their inventory page only shows me the FBA fees and I didn’t include the referral fee. This leaves no margin almost considering the cost of the item and the storage fees and I’m losing money every month. Any advice would be much appreciated.

Thanks