When the product BACKLOG is being prioritized, there MIGHT be some factors which come in way of doing it effectively. To list out some, first can be, the time needed for COMPLETION, though the item is on high priority the development needs time to complete it and it is not fitting in a sprint. In this case, the product owner has to grill out the most important part of the requirement to be shipped first. 

Then we can have, Correlative or conditional relationship between urgently required tasks and other tasks. There MAY be dependencies between the urgently required tasks and other tasks in the pipeline. The team cannot deliver the prioritized requirement before resolving the dependencies. Another one can be, timeframe given by customers for feedback is not ENOUGH for the teams keeping in consideration the slippage. Even sometimes the customer emphasis is too much that the product owner has to guide the customer on what market needs are and how to get maximum return. Even the customers sometimes need direction to follow, this is where the product owner can pitch in.