My child can read the words but does not remember or understand what they read. Why?

Reading is two jobs at once: turning print into words, and making meaning from those words. When sounding out still takes most of a child's effort, there is little attention left over to follow the story, so comprehension looks like the problem when decoding is the real bottleneck. For other children the words come easily but vocabulary or attention is the gap. Which one it is changes what helps.

What did you see in your house, and what moved it: easier books so the words came free, rereading the same passage, talking it through a sentence at a time, listening while following along? What made no difference at all?

When decoding is still hard work, comprehension usually rises on its own once the words stop costing so much, which is why we keep Lillory practice right at a reader's level instead of pushing ahead.