You are missing whether LinkedIn and mailers are relevant to your target customers and also how they use these media, if your targeting was accurate.
- (Buyer Personas http://j.mp/1LPsR1a)

GOOD NEWS.
Many CEOs of small companies are active on LinkedIn and even write their own content, but I can't help you without knowing which sub-segments you could serve better than anyone else.

You can even use LinkedIn to rank CEO's and categorise them by industry, company size and frequency of use, along with other criteria.
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SELECTING A MARKETING MIX
The better tactical selection will depend on what type of companies your CEOs run, what they expect from a vendor like you and where your target customers are, even if this means websites, phones or apps.

CEOs could be more receptive if they thought that you understand their point of view better than anyone else. Perhaps you could get attention from active target members by commenting on their content, (LinkedIn and anywhere else they write) because marketing is about your target, not you.

MEDIA PLANNING
- (Media Planning http://j.mp/1EByAbz)

There are many other variables that will interact along your target's decision process, like influencers and competition. I will only comment, however, on your mix of LinkedIn adverts and a mailer.

1.
(LINKEDIN)
Your LinkedIn adverts could have been poorly targeted and designed, because you didn't learn your audience's media usage before hitting them. Perhaps your CEOs are more active elsewhere, like Twitter, Facebook or Clarity,
This is why I need you to elaborate on your target segments or I can research it for you.

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HACK!
If you've still got a list from the mailer, you can check individual names on LinkedIn and interact with them. It would be wise to get a premium plan, at least temporarily, so that you can contact decision makers directly.

You can still use the post office for thank you notes and seasonal greetings.
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2.
(MAILER)
If the purpose of your mailer was to generate sales leads, the list to which you sent your creatives should have been made of prospects who were most likely to be shopping, based on their past behaviour and not only on demographics, which is often the case when you buy a postal package, but do not control the list.

There are many things that could have gone wrong with your mailer and LinkedIn ads each by themselves and many more by combining them. You can't either expect one round of anything to build you an empire, because sales are built on reach and frequency.

3.
(OTHER MEDIA)
Have you thought of other media, like events?
- (Media Planning http://j.mp/1L9U2kw)

Speed counts. Your competitors could read this answer and get to your buyers first.

Your options are:
1. (SLOW!) Exploring the links above for content.
2. Calling me to start your next marketing plan.