Yes, it can be hard to take someone you don’t know and to wish them well. Because that person doesn’t really exist for us as an emotional being, there’s not much to work with. The neutral person can be as elusive as mist that slips through our fingers.
But it will come with practice. Keep working at it both inside and outside of meditation, and you can find ways of working in this stage.
One initial problem can be expectations: we can expect the Metta Bhavana practice to be a sort of emotional firework display. The trouble is that in the third stage we discover our matches are damp! So get used to the fact that this stage might take time to develop. Look for a sense of acceptance that change will come at it’s own rate if you keep working persistently and kindly.
This stage of the meditation practice is actually meant to be at least slightly boring! What we’re learning to do is to cultivate an interest where none exists, and in order to do that we first have to acknowledge our lack of interest. The subjective experience of a lack of interest is boredom. This stage of the practice is therefore meant to be boring. QED.
Get used to just sitting with the image of the neutral person, while repeating the phrase “May you be well, may you be happy, may you be free from suffering.” From time to time you can break from that and try some of the ideas you’ll find in the page that discusses the third stage as rehearsal.
You might be tempted to keep changing the neutral person until you find someone who’s more interesting (i.e. not really neutral!). The restless mind always assumes that the way to “fix” boredom is to find something more interesting to do. But actually the way to get rid of boredom is to be more mindful and to learn to see what’s interesting in ordinary things.
I have a young nephew who has (in my opinion) way too many toys. What happens is that he sits surrounded by all of these gadgets, many of them very expensive electronic toys, and complains of being bored. I think this illustrates that boredom is a state of disengagement, and not a lack of a suitably interesting object. In some ways there is no such thing as an interesting object, just object that we take an interest in.
So, to get back to the meditation practice, it’s probably better to stick with the same neutral person for a good few sessions of meditation to allow yourself time to develop more of a feeling for them.