Answer: Bulb lilies or Lilium should increase each year and can be left undisturbed for years until they seem crowded and blooming diminishes. When the plants seem to just dwindle slowly from year to year, it is usually caused by too little sun (most kinds of lilies need at least a half day of direct sun), lack of fertility (lilies like a rich soil)n or possibly due to being planted in a heavy clay soil or in a poorly drained location leading to the bulbs rotting during the winter. Another cause for decline is lack of foliage to help rebuild the bulb after blooming. If disease causes the foliage to drop early, or if you cut the flowers with much of the stem/foliage intact, this limits the bulb's ability to rebuild its energy. Finally, some varieties are just more robust than others once they are planted in the garden. Deadheading or removing the spent flowers to prevent them setting seed can help with that to some extent. I hope this helps you troubleshoot.
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