Turn on paging or you only get the first page of a forum thread. Set maxPages deliberately: a big value concatenates the whole thread but burns one call per page. For cost control, cap maxPages to what you actually need.

Context: Official docs (Diffbot Discussion API): documents that paging must be enabled explicitly, that maxPages controls how many pages get concatenated into one result, and that each crawled page counts as a separate API call against your plan.