Lemon Balm Losing Scent

I have what seems to be a very strange problem. I've asked everyone I can think of, (even the Pennsylvania Horticulture Society), and no one has ever heard of this.
I have an herb garden, I live in Philadelphia, and this is my garden's 4th year. Every year, my lemon balm comes up beautifully, and this year is no exception. It looks beautiful, but the problem is that this year it has almost no scent. I use it to make tea, and usually the whole house smells of lemon when I make it, but this year there is almost no scent and no flavor. I have a second area which I planted last year, and this year it has even less aroma than the original planting. I just tried adding compost around the plants to see if that would help, and the man at the Horticultural Society suggested that maybe it just needs more warm weather.
I hate to think of pulling it all up and replacing it, without knowing what is causing this. Do you have any other suggestions?

When plants are grown too soft and too lush and nothing is stressing them, they don't need to produce the scent, because the scent is supposed to deter munching insects and other animals. It just does not work for us, because we happen to like the scent and flavor. Most plants have at least some enemies that are not impressed by their defences and that stops these plants from covering the whole earth with their progeny! We utilize many of these defensive chemicals for medicinal purposes - and warn users of the dangers of using them improperly.
So start to stress your two plants a bit. Brush your hands over them frequently, hold off on the nitrogen. Increasing heat as the season advances will do it too, because this plant prefers moist, cool conditions. Just don't overdo it to the point that you kill it! It is best to stress the top of the plant, but not the roots.