Not enough power, particularly in the lulls? Make sure you have sheeted the jib on properly to ensure the top luff tell tale is flowing. Make sure yo have pulled the boom back to centre, eased cunningham and rig tension (and maybe vang).
In this photo the boom is not pulled in all the way so the crew cant lean harder and harness the wooer available.
Every time the crew needs to stop leaning the boom should be pulled in hard so crew can continue to lean as hard as possible.
This requires constant communication between helm and crew.