Ah, that's really helpful somewhat. Although my post plug plants of antirrhinums and lobelia are just about ready to plant out, my begonias seem to have come to a standstill. I have given all of them a dilute feed of Tomorite as I potted them up in John Innes No. 2 so thought they might need a little boost in case they'd run out of nutrients. Can't explain the difference between them though.
I find they don't do well just sown into compost & put on a heated tray. I don't think the actual compost gets hot enough for them done like that. I put them on a big wad of folded kitchen roll, very damp, slide it into a freezer bag, fasten the top of the bag to keep moisture in & then lay it directly on my heated tray. They make roots in just 3 or 4 days. When the root is about an inch long I transfer to compost & they grow away well. Hope that helps.