I suck at direct-sowing, and tend to have the same problem with almost anything I sow (not entirely random patchiness of rows). I attribute it mostly to unevenness of soil conditions -- some parts being more compacted, or less effectively covered to keep the sprouting seeds from drying out, or that sort of thing. I dunno if this is *right*, but, it's what I believe <g>
If it bugs you, you can sow an additional small area between rows that can serve as a transplant source. You can just step over it for the first few weeks, then 'mine' it for transplants, then it goes back to being between-row pathway cuz no plants left in it anymore
Good luck, have fun, nice lettuce,
Pat, whose garden has been so flooded for so much of the spring that virtually all the lettuce I have is in 6 windowbox containers, sigh.