There are many factors which leads to failure in order to get positive ROI.

1) Marketing campaigns showcase products and provide exposure to more users to gain popularity or attain visibility.
---- Its required that product/services for which agency run campaigns are having basic characteristics which the attracts the right audience.
**** Sometimes due to lack of product knowledge or getting the campaign run for right audience fails leading to negative ROI.

2) Being a Project Manager I think Project and Business objective may not be communicated well to Internet advertising agencies that leads to false assumptions and ultimately bad execution of marketing campaigns.

3) Competitor's Campaigns may over-rule your campaigns and diverting the user-base interest to your competitors.