Hi there,
It's good to consider how success in one area can contravene another. In terms of diplomacy;
-2007, China's veto of intervention to stop 'political repression' in Myanmar. This helps to achieve national security, as it may prevent US influence in the region - you can develop this point. However, this may IMPEDE regional relationships - the international image of of China's 'peaceful approach' begins to deteriorate, states in the region become include to consider the states devotion to its own national interests.
-2013, denouncement of Japan's growing militarisation - think about how this could cause strained relationships but assist in promoting positive international standing or 'peaceful rise'.
-Military power - there are sooo many negative things you can talk about here. Everyone knows about the 1,600 missiles aimed at Taiwan, but this really does help to protect sovereignty/prevent separatism, yet it severely contradicts regional relationships once again. You don't need the example to be 'unsuccessful' as such, you just need to think about how this success may actually cause tension in another area. Therefore, if you're confronted with an 'evaluate' question regarding the achievement of national interests, you can say 'China's veto of intervention in Myanmar allowed for the prevention of democratisation in the region and increasing US influence (blah blah), yet this may be considered an unsuccessful example of achieving 'positive regional relationships'. Don't think you have to write about a failure!
Please message me if you would like some more info, happy to help.