Unable to get the buy box with FBA

Hello,

I am a fairly new seller on Amazon, I have had an account for almost a year now but have only decided to use it the last couple of weeks.

I have sent 200 units to FBA as a test run, now my listing is showing as buy box ineligible

1. I have a Pro account

2. I use FBA

I don't understand what else I need to do, I have followed the guidance provided by Amazon, I have the lowest price and stock is available for next day delivery