Hi Danh Lee,
Asking visitors is a really good method of finding out why they didn't purchase immediately (same suggestion as @Rockpapercopy). It might be the case that they were shopping for someone else or they weren't ready to buy today. So just because you don't get immediate sales - it doesn't mean that you wont get any sales. Maybe introduce live chat to your website too. It might be worth considering live chatting to visitors whilst they browse your website to assist them too.
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