When you say "small clusters", are you seeing clusters of plants?  or each plant looks like a cluster?     How far apart are your individual plants?  I think you want to see individual plants at least 8" apart.  Here is a link to a gardening website, and they suggest individual plant spacing at least 12" apart.

Frankly, I have planted head lettuce, and still harvested individual leaves to eat, harvesting from the bottom of the plant.  If you are wanting some lettuce to eat now, I would go ahead and start harvesting bottom leaves from some plants, and leave some plants to fully develop heads.  Lettuce is going to grow until it bolts and sends up a seed stalk.  That is what you can expect.