Sales and marketing efforts help acquire customers while product retains. To understand where the issue lies, I'd start with assuming there are product concerns and prove otherwise.
I'd start with the retention metrics for your product, namely churn. Do you have churn? Is churn related to issues in the product vs issues such as credit card expiration or misfit customers? Certain churn is acceptable for the type of industry and model you play in. We are a Saas based company and churn up to 2.1% is acceptable. Since you have customers, this is a number you should have data on.
If you have a product that's been in the market for a while, you can also look at CLV (customer lifetime value) and determine average CLV for your product. Product features and experience can have a direct impact on increasing CLV. Having a high level view of your average CLV and analysing them month over month can help you determine if customer are churning because of product blocks.
But the best way to get direct insight into your product is to talk to your customers. For a small startup like ours, I sent out monthly surveys and talk to our customers to understand how they experience our product. You can also get insights into product features that are the most valuable to them and features that have little value. Use this data to ensure marketing campaigns and messaging highlight the highest value customer benefit.
If these numbers look good and above average for your industry and business model, then the chances are good that your product is not the problem. Hope this helps!