That is usually because an ad is dynamically allocated to a slot (of a dynamic size often too), which starts after the page loads. If said ad is above the conte

That is usually because an ad is dynamically allocated to a slot (of a dynamic size often too), which starts after the page loads. If said ad is above the content you want to interact with, it pushes everything down (below) and there you go. It can be resolved by a) not using dynamically allocated slots or b) by never putting ads above any content someone might interact with.