A lot of Our FBA Inventory keeps being moved from 'available' to 'reserved'

We have sent lots of inventory products to FBA Amazon keeps being moving chunks of it from ‘available’ to ‘reserved’. Consequently sales drop because there is hardly any products left in the available inventory. It is very annoying because we have money tied up in the inventory and we are loosing sales because of it. It has happened several times in the last few months. When we contact amazon about it we dont get any sense from them.