Dwarf Conifers

I'm curious as to why you put a layer of gravel in - if your soil is clay, its not unusual, and is beneficial, to add sharp grit to the soil, but not in a layer like you describe. You don't say what depth of topsoil you laid over this layer, but I'd be inclined to say you should perhaps redig the bed, mixing in the gravel layer with soil, and then replant with dwarf conifers. I know you're in the States, but I don't know where, so it may also be that the ones you choose to plant aren't hardy enough for your Zone? Is that possible? Otherwise, I'd take a guess and say the conifers hit your gravel layer and give up the ghost immediately - or that the gravel layer encourages water to drain too freely, and they dry out, which conifers do not like to do. They like moisture retentive, but not wet, soil.