The autocomplete suggestions are currently based on wikidata terms (labels or alias) where the beginning matches the search string.
This prevents getting suggestions when searching for multiple terms.
E.g. you might be looking for "flag russia" (and would expect the latter word to get autocomplete suggestions), but because there is no item with "flag russia" label or alias, it doesn't autocomplete.
Including a similar search for entities based on only the last word and including those in the autocomplete results, would probably be a good improvement, even though it might result into suggestion combinations that don't make a lot of sense (but then again, this also happens with Google autocomplete)