First, you have to define what exactly is your marketing department composed of :

1. Do you have technical writers, market analysts, product strategists, branding experts, graphic designers, advertising experts ?
2. What specific function of your marketing department is failing or stagnant ?
3. Are they failing to promote your products/services effectively ?
4. Are the failing to follow the competitor and market trends ?
5. Are unable to bring in good ideas for existing and new products ?
6. Are they failing in effective branding of your company/products?

Marketing department these days is like the backbone of a company. In fact it's even critical to the other two core parts of the company, i.e. the sales and product team. So they need to be very passionate and active, and help push the company forward. Stagnation can be due to several causes e.g. :

1. Apathetic employees or team
2. Poor or inexperienced leadership (for the department)
3. Weak/New Industry, Poor/New Brand Name, Unmarketable Products (or difficult-to-market products)

Outside firms are not cheap and not sustainable long-term (unless you can afford it). If you plan to do some one-time or short-term projects, like getting consultation on a new project, get a new website design, doing brochures, exhibition, advertising, some branding advice etc.. then that's alright.

In the long-term however, you will still need a set of skilled and experienced people in your marketing department to help push the company forward. Also, doing this internally will give you better control and flexibility, and you can eventually accumulate a lot of know-how and experience in terms of marketing (that is very relevant and specific to your industry).

I would recommend an in-depth analysis of your current marketing team and an optimization plan. Sometimes all you need is one genius, one fantastic marketing leader, who will turn the whole department around.

This requires an in-depth analysis of your business, products, competitors, target market, revenue model, expenses/costs, sales model etc.

You can get general advice from anyone, but better advice from someone who is an expert (more specifically) in the product or services you provide.

Talk to an industry insider, to understand what's wrong in your company, and what needs to be improved, and whether it can be improved or not.

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.