Yes it seems to be quite tricky to grow from all of the lettuce types. An alternative you could try might be a batavian lettuce like rouge de grenoble, it's halfway between a cos and iceberg in terms of texture.
I don't grow lettuce for hearts as I always find a slug in them which is most off-putting. Instead just for leaves to pick off. Romaine/cos type lettuce are easiest for that, but you can also do it with hearting lettuces too. It also makes things easier for sowing, instead of having to sow every couple of weeks to get a steady supply of hearts, you only need to four or five times a year for a full year's supply. We'll I would normally except my last lot suffered sluggaddon just after planting them out and they got decimated , so I've had to sow again and will miss a few weeks supply of lettuce rather annoyingly.
