When I put the price down listing disappeared

Hi Guys

I got a product I’m selling since last October. I sell about 10 items per month . The value of the item is £75. Every time when I put the price down, I’m losing the buy box or it says pricing error and the item disappearing
I send a few emails to Amazon seller support and they keep sending automated messages with a loads of links.

Can anyone tell me how to put the price down by £10 at least on Christmas period, please