That sounds pretty deep to me - I haven't planted any new pips in years, but the growing points of mine are always just visible at ground level. The plants go dormant in July and August when our heat and humidity start to rise, so you might want to be prepared for that; they seem to be more of a northern plant, but they do bloom pretty well here every year in the shade.

I also think that is too deep for lily of the valley. I doubt they will have the reserves to reach the surface and produce leaves. Try digging a couple and see how they are doing. Al