Such low conversion rates are typically because there is a disconnect between the ad and the product.

Without specifically knowing what your app is about, there can be multiple reasons why people are not downloading the app

1. The ad promises something, but on the app description, that promise is not reinforced or highlighted.

2. You have bad reviews and low ratings.

3. The app page doesn't have well written descriptions or are lacking in screenshots.

To address this situation you can do a few things:

1. If it's bad reviews/ratings then address them. Improve the user experience.This is the most likely reason why people are skipping the download button.

2. Improve the description etc on the app page. Give users more information. Align the ad with the app.

3. Run a re-marketing campaign. Run a different ad reiterating your USP and show it to those 25k people who have visited your app page.

I hope I have given you some ideas, and I would be happy to take a look at your ad and your app. if you want me to be more specific.