Sample products not working is not a good sign, and typically unusual. Normally the samples always work perfectly. It's usually the subsequent shipments that have issues in quality. I don't mean to say this applies to every Chinese company, but it does happen quite a lot. Obviously companies in China are becoming more and more professional, and it's no longer difficult to find companies or factories that do very professional work. However, there are many companies there who look for a quick buck, and since they have an unlimited source of potential customers globally, they don't necessary care if they lose their customer, after an initial order. That's essentially how some companies in China earn money on platforms like Alibaba and Taobao. They showcase a product that looks and sounds amazing, until you get the actual product (typically in higher quantities) and find it to be different than advertised, or of a much lower quality, or lesser features, or not functioning at all. They are not expecting return customers in most cases. Most of their money is earned by first-time buyers who haven't researched the company or the product well enough. I know this sounds like a scam, but truth be told, China does have many such suppliers and it's not easy to spot them if you are working with China-based vendors for the first time. Also, keep in mind, that as a foreign buyer (not based in China), there is very little you can do (legally), in case you have been tricked or cheated by a Chinese supplier. My recommendation would be to first do your research thoroughly on the company and their product. If you are unable to do that, and also unable to find a local representative/agency to verify this company's history and quality, then the safest option would be to work with this supplier on credit/payment term basis, where they ship the product first, you verify all the units are functional, and then pay them within 30,60 or 90 days. Another safe option is via Bank LC (though beware that many Chinese suppliers refuse to accept this and prefer advance TT only).

PS : Samples usually always work. They are either very professional or very unprofessional if half of their first samples were not be working. I would advise caution.