Each voucher and event has its own unique conditions which sometimes are not compatible with each other. That's why, when applying one of your vouchers in a purchase, they will be divided between “Available” and “Not Available”.
If you want to know the condition of the voucher, you just need to click on it: you can then see it's expiration date, compatibility and limits.
On the other hand, if an event has restrictions on how the vouchers can be used, you'll also be able to see this in the breakdown prior to the purchase confirmation: there you can see if your voucher and discount has been applied properly.
To do this, all you need to do is go to the "Vouchers" section and click on your voucher. Here you can see what, when and how you can redeem it.
To learn more about how to use vouchers in your purchases, check What are coupons and how can I add them to my account?