Answer: Lily-of-the-Valley (Convallaria majalis) is usually quite trouble free, but there are several possible answers to your question. The older bed may simply be too crowded, causing the plants to be slightly less vigorous. You might try thinning them. Early spring or fall is a good time to do that, especially if you want to start yet another bed.
There may be a nutrient deficiency as you suggested. Although this plant is not terribly demanding in terms of nutrients, after twenty years or so it is possible that the soil has become somewhat depleted. You might try a top dressing of compost or other organic matter along with an application of a balanced fertilizer to see if that helps.
Another possibility is that the dry summer last year affected one bed more than the other. This could be due to either crowding, competition with tree roots, or the general drainage pattern in your yard.
Finally, if you obtained the pips for the beds from different sources, you may just have two very slightly different varieties.
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