Joe, 70 to 75'F is too warm. Much better to be 65'F. With some crosses you might have to go even lower. What I do with pots that are 'no shows' after 6 weeks is stick then in the fridge for 3 to 4 weeks, then bring them out for a second try. If the room temperature is too warm (above 65'), I find a lower step on the basement stairway that's reasonable. Many times that will produce mixed results with quite a few sprouting. Some seeds that don't sprout this way will sprout late in the fall when the temperature drops into the 40's and 50's.
I had 4 stubborn batches this Spring. Only two seeds sprouted between them and they were probably the result of stray or tramp pollen. After a 3 week chill in the fridge, one pot is now bobby pinning with 7. The others, nothing yet. The seeds are swollen, but they just won't budge. I'll bet you a coffee they'll come up around the end of October/early November.
I usually cover my seeds with no more than 3/16 of an inch of media, mist heavily, leave open overnight, then cover--which helps maintain a moist surface longer. Also lily seeds don't need light to germinate and lights often radiate heat which can raise the soil surface temperature 5 or more degrees higher than the room temperature.
