What's the biggest problem you face when A/B testing your funnel?

A/B testing is supposed to systematically isolate winning and losing acquisition funnel content.

My biggest struggle, by FAR, is understanding why people bounce from my landing page after clicking through my ad.

If they don't bounce, and enter the funnel, it's much easier to test why they don't make it to purchase. But isolating the reason for a high bounce rate is tough.

Have you faced this problem, and how have you managed it?