It's been a cold and somewhat damp spring this year. Last October I planted tree lilies, planting the bulbs about 7-8" deep. It's May 5th and I haven't seen any sprouts. Is it because the soil hasn't warmed up enough? There are no signs of animals digging them out.
I'm not sure what zone you are growing in.
Tree Lilies are hardy in zones 4-8 and can tolerate the warmer temperatures that are in zones 9 and 10.
The ideal depth for planting your Lily bulbs is 10 to 12 inches deep.
If you are in a cold zone, and the bulbs did not have enough winter protection and insulation, they may have froze.
Give it a few more weeks, it is still quite early. If you see no signs after that time, I would dig up one of the bulbs to check it's condition.