Why don't my lithops ever bloom?

How many hours of direct light are you giving them a day? Because that's the problem nine times out of ten. Lithops need a ton of sun - like 6-8 hours minimum of direct light. If you keep them indoors behind a regular window or in partial shade, they won't flower. Period. Try putting them where they get the strongest sun you have available, maybe outside in summer if you can.

The other thing is the winter dormancy. They flower in fall-winter (depends on the variety), but only if you've let them "sleep" during the right period: less water, cooler temperatures if possible, no fertilizer. If you keep watering them normally and keep them warm year-round, forget about flowers. During dormancy they need to be basically dry - only water when they're completely dried out.

Third thing: age. If they're still young, maybe they don't have the strength to flower yet. Plus, if the soil is too nutrient-rich or too moist, the plant grows but doesn't put energy into flowering. Use a super-draining substrate (almost like sand, with very little soil), and fertilize very little or not at all. They kind of like to struggle a bit.