I'm not a daylily grower, but grow others seeds. Vermiculite has little readily available nutrients in it so unless you are providing some sort of dilute fertilizer (around 1/4 rate [I make it even weaker the first few times that I fertilize seedlings just to be safe]) then plants will stall. You do have to be very careful not to burn seedlings with too much fertilizer. The nutrients in the seed get used up pretty quickly. Photosynthesis produces oxygen (released) and glucose (for energy) but a plant needs other elements (nitrogen, phosphorus, potassium, etc) to actually create more cells and maintain physiological processes. If there are none of the required elements in the growing medium, then the plant can't do much growing.
For the one that you potted in soil and fertilized, the constant rain may be leaching the nutrients away faster than the little seedling can absorb them. That's just a guess , though.
