After you make a combination you have to adjust it's quantity.
There's two ways:
1. Simple and stupid (for small shops) - you don't use advanced stock managment - the quantity field in your BO product page is editable because you don't use ASM in your shop preferences and you don't have choise to enable it in your BO product page -> Quantities. You enter your stock manually, the shop deducts it with the sold numbers for each order.
2. Little more complicated (for shops with multiple warehouses, or suppliers disguised as warehouses) - you use ASM - you have it turned on in your Preferences -> Products page, and you have created default warehouse (this have two steps - you have to create the warehouse itself, set it up, etc and also you have to assigne carriers that will serve it and this is important because otherwise the customers won't be able to checkout, the system will give them 'No carriers available for this product' message. So you have your product and your warehouse. In BO Product->Quantities tab you select 'I want to use my new and shiny ASM' and click on 'my stock is there please use the numbers already and leave me alone'. So the quantity box will grey out. After you save your product you go to Stock->Stock managment find your product and fix it's quantities (you have different options here aswell).....
If you have 1 guzilion per item you can simply funk off all this shit and check When out of stock - allow order in your preferences and leave all this stuff behind you, pour a glas of wine and relax...
In simple words - your site thinks you don't have stock from those combinations and hides the BUY NOW button. If you put stock in the system it will show it fine and dandy... :) Until now all of your products had no combinations and you had put some quantities somewhere but when you create a combination for those products the system does not keep the quantities you put earlier becase it doesn't know which goes where and simply leaves the admin to redo them. So that's why the button is gone.