Bok choy (Brassica rapa var. chinensis) and caixin (Brassica rapa var. parachinensis) seeds are hardier than lettuce seeds because of their greater tolerance to temperature fluctuations and various soil conditions. Although generally easy to germinate, lettuce seeds require more specific conditions and are more sensitive to temperature and moisture variations. This makes bok choy and caixin more adaptable and easier to grow, particularly in less controlled environments.
However, the sowing medium might be contributing to your germination problems. If you have been reusing the same medium without replacing it, it may have become overly wet or contaminated. To reduce the risk of diseases and waterlogging, a clean or sterile seed-starting mix should be used instead of garden soil.