Members of the lily family require a minimum of 5-6 hours of full sun to bloom. If yours are under a mature oak tree, They are probably not getting enough sun. Unless the lilies are in containers, the tree is robbing your bulbs of both nutrition and moisture.
Have you been fertilizing the flowers? Also try using a more basic potting soil and make sure the plants receive sunlight. Lilies are generally photoperiod flowering plants meaning they depend on different light spectrum between the vegetative and flowering periods and they need different exposure times to the light. Good Luck and Happy Growing!