When you call your friend to mind in the second stage of the meditation practice, be emotionally truthful. If you don’t feel much at that particular time then that’s fine — it’s just where you’re starting from.
It’s quite normal to feel a bit neutral, and you might even (sometimes) feel some irritation towards your friend. These things happen.
Your love (metta) for your friend is still there, somewhere, and you can still strengthen that love in meditation even if it isn’t immediately apparent. In these cases the practice helps to put us in touch with our true feelings for our friends.
Often what happens is simply that the situation we’re in is a little artificial. I mean we’re consciously sitting there calling a friend to mind, perhaps wondering if we’re doing it right, and trying out the improbable task of cultivating emotions. It’s no wonder we’re sometimes a little self-conscious and stiff and that things don’t “flow” right away.
The worst thing to do is to try to force some kind of feeling to arise. That won’t get you very far — it’s better to look for a sense of patience — remember that developing metta is like growing seeds. Forcing a feeling to happen is like trying to pick the seed apart to make it grow faster.
Be patient, be kind. Allow your feelings to develop at their own pace as you continue practicing this stage of the meditation.