Heat is the most likely culprit. Select seed sources of more heat tolerant varieties, paying attention to heat tolerant seed which is prone to staying sweeter rather than just avoiding bolting in the heat.

Agree. Too much heat and too little water.

Dave

And perhaps also going to seed?

Heat and water. Especially water. I found that even when the weather is cool my lettuce will get bitter if not given enough water. Fortunately that bitterness (from lack of water) is reversible in a day or so after a good watering.

There is another thread about overwintered lettuce currently where the idea of bitterness being caused by age has been mentioned.

I grow lettuce here in Houston. I've tried about 15 varieties over the past 3 years. I do my planting in October usually and harvest leaf by leaf until around April 1. You'll probably want to plant a little later down in South Texas (RGV?), and you'll probably finish sooner.

Anyhow, the varieties so far that have done the best for me are Black Seeded Simpson, Crisp Mint, Gentilina, and Mignonette Bronze. They seem to roll through the occasional winter heat wave without bolting or turning bitter. Even those don't last once it starts hitting the eighties for me, although at that point they're pretty old. I started a second round in January this year, so I'll find out how hot a younger head can take before it turns bitter... Crisp Mint I especially like, because in addition to staying sweet, it stays pretty crispy.

Water is crucial, as others have said...