What are "Impressions"?
It is the amount of views your sponsored product is receiving within the Search Results. This is not equal to Search Volume. If your sponsored product only appears on pages further behind, it will not get any impressions.
Possible Reasons for Low Impressions:
• Wrong Sub-categorization of Product
In many cases, Amazon may not deem the category registered for a product as fitting enough. In such cases, it is advised to check which lowest sub-category your main competing products are part of.
• Bids not High Enough
It may well be the case that other sellers are simply bidding more and thus have their products shown more often on the front pages of the search results. Amazon's "suggested bid" is in many cases slightly optimistic.
• Budgets not High Enough
Amazon prefers keywords which have already generated clicks. This can often lead to the scenario that only few keywords will continue to be displayed and get clicks, so that in the end, no budget remains for the other keywords.
This is especially notable for (exact) ad groups where you only have very relevant keywords. Therefore, it is recommended that you increase your budgets.
• No Impressions / Clicks / Sales History
As mentioned, Amazon will always prefer products that already have a certain history for a specific keyword. Therefore, if you are starting to place ads for a new keyword, you may have to increase your bid over-proportionally at the start to generate some first impressions and thus clicks.
According to many sources, a good Click-Through-Rate (CTR) is the most important factor for a higher PPC ranking, as it is, of course, in Amazon's interest to generate as much revenue as possible from their PPC Platform.
• More History in other (Automatic) Campaigns
It is for the same reason that manual campaigns often receive no or fewer impressions than automatic campaigns: If a specific keyword is still active within two campaigns, Amazon will probably choose the ad of the older campaign which has generated more based on historical data.
Therefore, you would need to add the respective searchterms as negative keywords in that older campaign, so that the ad of the new campaigns will start displaying.