Woah, those Sydney summer temps have been too high for happy germination. I planted some candidum (from the same source) about 4 weeks ago and it started coming up a week ago. Over a couple of years of experimenting I've found candidum is happy to germinate here in the autumn and is fine through winter. I just can't get it to flowering size.
If my summer temps were so consistently high I'd be tempted to do all my epigeal planting in autumn and winter. Sydney winters are around mid-teens usually? That would be perfect. I should have thought of that sooner, forgot just how much warmer (and more humid) it can be up there. Sorry.
So collapsing seedlings... I'm thinking fungal problems. Damping off or botrytis getting hold from the get-go. Goodness knows what, but with higher temps (both Melbourne and Sydney) and higher humidity it's highly likely. Also, over-watering is the most common trap for first-time lily-sowers, even if we think we're being cautious we tend to over-water rather than under- and if any babies are too moist they will quickly succumb to fungal problems.
Mind you, you're right Protoavis, that some seedlings are just genetically weak (or not suited to the conditions they find themselves in).
I've had a wretched seed season this year. Firstly I tried another new potting mix and the lilies hate it. Too late, hah - I filled and sowed scores of styrofoam boxes before I had an inkling... but secondly we had SO MUCH SPRING RAIN the poor things that didn't drown, starved. Surviving seedlings have just sat there, gone yellow and purple and sulked and I haven't found anything that can remedy the situation. I'm not up to the task of transplanting thousands and thousands of rice-grain-sized babies into new soil. Last time I had a batch of mix that stunted seedlings it took a couple of years for it mellow, then the lilies grew as normal. I guess that implies some kind of allelopathy caused by under-composted plant matter?
