It may be the signing up process. Do you require your customers to register to your website by filling in certain info like name, number, etc?
The best way to do so would be to allow people to log in via Facebook and that would allow you to get those simple details such as name, email, etc.
I have found that people tend to leave the buying/conversion path when they have to enter such details because it seems like such a laborious process. Therefore by using the Facebook login button you eliminate that laborious process and all they need to do is enter their delivery information. Follow this
link for more information:
Alternatively people may be against paying via credit card, you may want to look at alternative methods of online payment such as purchasing vouchers from Pick 'n Pay, using Pay-Pal, EFTs, and so on. Check
here for more details.
Hope this helps.