Hi Maria,

PLA (Now known as Google Shopping) :-) can increase Conv rate by applying different method - for instance by adding negative keywords. If you, while logged into AdWords and inside your Shopping campaign, go to keyword level, you can see all the search terms that triggered an ad showing. See if you can identify search terms, that are irrelevant, and then add them as negative keyword.

It can be a great idea to add generic words to your negative keywords list, especially in your shopping campaign, as you can not quite control which products are being shown to the different search queries.

Check to see if all your best converting products has got a search impression share as close to 100% as possible (spend money on the products that are converting = better overall conversion rate)

The ad is not the main driver of your conversions. Your landing page is. So you can't blame Adwords if you are not getting conversions. Ads are there to create an interest and curiosity. It can't by itself make the sale. Yes, it can help steer someone toward a sale, help pre-sell and place that thought in the reader's mind. In the case of PLA, the image can help. But in the end, it's what you say on that page that has to convince the visitor and close that sale.

For PLA, your title is important. It should be what the product is and be keyword-rich as many put it. Your description also since PLAs will be triggered by keywords there. As said, the image can have an effect. One often forgets about negatives when it comes to PLA so think about them. You also need to structure the campaign properly, not just put all products into one basket.