If your child reads well but writes weakly, the bottleneck is probably in production rather than comprehension. Reading can supply language and ideas, but writing also requires the learner to retrieve words, build sentences, organise meaning, sustain a line of thought, and edit while keeping the task in mind.
Check whether the child knows what to say but cannot phrase it, writes sentences that do not connect, uses vocabulary passively but cannot retrieve it in composition, or loses control when time pressure begins. Those are different problems and should not all be treated with more reading alone.
Use the English Learning Hub for writing, grammar, and composition routes, and the Vocabulary Hub when word retrieval or productive vocabulary is part of the weakness.