What seems to be wrong with my automated Lettuce farm?

Solution 1:

Bit old, but the likely issue is that you don't have a secondary item output. If all you have is harvester->pipes->planter, and you can't send the lettuce heads anywhere, things jam up. The reason carrots/potatoes work is that the plant is the same as the seed, so it can just send everything to the planter.

You'll want a chest to output stuff to, making sure the planter still has priority for seeds.

Solution 2:

Since the harvester can only really do vanilla crops, I used autonomous activators on each of my lettuce plants with an itemduct connecting the output slots to a JABBA barrel. You can choose whether the autonomous activators right or left click, so by having them right click you don't have to worry about replanting the lettuce seeds. Yeah, it's expensive and takes up too much space, but it works pretty well seeing as harvesters aren't compatible.