A student in a project management class I taught shared the concern that it was very hard for her to get risk responses implemented. This is a fairly common problem and is likely one of the reasons that the volunteers who updated the PMBOK Guide, Sixth Edition added Implement Risk Responses as a new process within the Project Risk Management knowledge area.
Most companies which have project management standards require teams to identify and analyze risks, but merely capturing information in risk registers is worthless if nothing is actually done to manage those risks. Acceptance is a risk response strategy, but project managers are not supposed to just report on accidents, they are expected to prevent them. But there's only so much that they can do by themselves. Risk management requires investment from stakeholders outside of the project team to really make a difference.
Getting a reluctant stakeholder to commit themselves to a risk response requires change management so let's see if Prosci's ADKAR® change model could be used as a framework to achieve this objective.
Awareness & Desire: If the proposed risk response owner is not aware of the need for them to participate, nothing will happen. Sending them a risk register by e-mail and asking them to review the risks which they can help with isn't likely to generate a prompt response. Meeting with them in person and clearly articulating the nature of the risks and the proposed responses might work better. Response owner awareness is a good starting point, but why should they expend their valuable time, money or political influence? Helping them understand how they have "skin in the game" for the project's success will be critical if you want them to commit themselves.
Knowledge & Ability: Does the response owner clearly understand what you'd like them to do and do they possess sufficient context regarding the risk? Do they already have the necessary knowledge to plan and execute the response, and if not, how can you simplify that learning curve?
Reinforcement: Just because you've had a meeting with the response owner and they've bought in to the need for their action doesn't mean that you can wash your hands of the risk. Regularly reporting on the status of implementing risk responses to your sponsor and key stakeholders as well as following up with response owners will be needed to increase the likelihood of follow through.
Read any of the case studies which are published in PMI's monthly PM Network magazine and effective risk management is nearly always identified as a contributor to a team's success on large, complex projects. But without addressing the need for personal change, your risk management efforts are likely to remain an academic exercise.