I think this has pointed me to the answer. I didn't feel I was far away, as the PET bottles were nice and firm, sounded right when opened, and the beer wasn't flat but just not as lively as I like. I've been pouring very carefully down the side of the glass, trying to keep sediment in the bottle, but a messy volcano after pouring from a pitcher (yes you did say carefully) delivered a pint with a convincing head. So for now I'm pointing my finger at over-cautious pouring technique. I guess I just need more practice....