paypal discount amount

if anyone else is using a paypal library with codeigniter and can use the discount_amount_cart I'd quite like to know which paypal library works ok and if there have been any issues with passing a discount amount (not a percentage as that seems fine)
I was reading on paypals deleloper site that tax_cart might cause the discount to be dismissed, but even with the tax_cart set to 0 it makes no difference.
As for as I can see, there are no issues with my code, paypal seems to just ignore the discount_amount_cart and I cant see why, Im stumped to be honest, there doesnt seem to be any reason why it isnt being passed over to payal, or not one I can see.
I've been using codeigniter for years now, it isnt a codeigniter problem as such, either Im missing something or the library has a problem or paypal are ignoring the variable, I can figure out where the problem is.

I've found the same issue with open cart and oscommerce when searching for a solution, nothing relevant to CI or the library Im using, anyone else able to pass a discount amount to paypal without problems?