There are a couple possibilities:

1) The perception you're creating with your ad, and the perception you're creating on the app store page are not matching up. So the type of people that are being attracted to your ad, are not being attracted to your app store presentation. Meanwhile, people that would be attracted to your app store presentation are never getting there because they're not attracted to your ad. This discrepancy between ad and store presentation could be due to many things including bad looking screenshots (or just different from what they pictured from the ad), bad reviews (or no reviews), etc.

2) There may be some inherent flaw in your product that doesn't appeal to humans in general. An example of this could be a cost that is far above what most people would be willing to pay.

If you'd like advice more tuned to your actual product, send me a link and I'll be happy to take a look at it.