Whether it is lettuce or flower seeds, I find I can almost mark my calendar at the 21 day mark to bump them up or plant them out. After 3 weeks they seem to stagnate. I wonder if they have used up all of the nutrition available at that point too? It's a puzzler.
And like you I am struggling with getting some lettuce seeds to sprout, and they are not even that old. Rouge d'Hiver and Winter Density seem to be my nemesis. I am on my third try with them. If I am lucky I get one or two out of ten, and the ones that germinate are really wimpy. I am toying with broadcasting the remains of the packet of each of these to see if I can get something to grow to maturity, so I can save my own seed from them.
Have you tried any of Frank Morton's blends from Wild Garden Seed? They are kind of fun, with lots of variety.
My technique is poking a very small indentations of about 1/8" in the mini soil blocks (usually with the round end of a chopstick), mist them well with water (or else I use a sports water bottle to drip a few drops of water), place a seed firmly on the soil in each 'indent' using a wet toothpick to pick up the seed (for good seed to soil contact it is supposedly important to press the seed in), and then oh so lightly dusting it all with very fine vermiculite to help keep them moist (Ferry Morse makes a nice fine vermiculite). Then loosely cover with a dome and leave it in a bright location at room temperature for a few days. Many of the varieties of lettuce that I have started this way are up by day 3, and leggy by day 4 if I don't get them under lights. Crawford Estates had really erratic germination; it is a new variety for me. Most just pop within days.