Make sure you have a clear indication of what you want to acheive, and the memory you are using. If these get cloudy, you won't be able to use them as a focus.
Also remember, that you are not trying to simply recall an emotion. You are adapting it into a new scenario. If the emotion doesn't come, consider looking at your script, and working out whether there is enough build up to warrant it. Usually it will have a gradual build. In real life, emotions rarely are a snap occurance, they are an accumulation of past events, leading into a release. Consider focusing on your recall earlier in the scene, so you can feel the build, releasing it at the appropriate time.
Also, try different triggers. You may find that some memories work better than others. Especially with strong emotion such as fear or depression, your cause may not be strong enough to create the effect.
