I live in Jakarta city, Indonesia, started planting the lettuce at September 7th, 2019 and today (October 10th, 2019) should be the harvest day. But it seems the lettuce growth have stunted since one week ago. I didn't measure the air temperature, light intensity, and air humidity during cultivation period, but in my place the average air temperature 30 - 35 ⁰C on the afternoon and 28-29⁰C at night, light intensity around 18 - 65 klux, air humidity around 45 - 55% in the morning and 80% at night. The electrical conductivity nutrition around 0.9 - 2.27 mS/cm. What caused my lettuce stop growing?
Growing lettuce in warm/hot temperatures requires very careful choice of variety. Generally lettuce appreciates cool temperatures and unless specially adapted to heat will cause either slow germination or premature bolting to seed. In addition your location has been hitting the news with respect to pollution, which may be having an adverse effect on the light reaching your plants. So there are two points to consider.