Display ads function well in print and on desktop computers. But there’s a growing consensus that they just don’t work on mobile devices. Here are three reasons why:

1. PEOPLE DON’T LIKE THEM: Surveys show that people find mobile ads more intrusive than desktop ads, because mobile is a more private venue. In fact, fully 1 in 5 say that mobile ads are “unacceptable.”

2. THERE’S NO RIGHT SIDE: PC users are conditioned to find ads in the right margin of the screen – they appear that way on Facebook and in Google search results, for example. But mobile screens are too small to have a usable right margin, so ads pop up in unexpected places.

3. THE “FAT FINGER” EFFECT: Advertisers closely track how many users tap on an ad. But many of those taps are inadvertent, because the ads are tiny – so it’s difficult to judge an ad’s effectiveness.