Phenolphthalein is a laxative. Therefore, you should keep track of which end of the straw you blow into and always blow into the same end of the straw. Carbon dioxide dissolves in water to form carbonic acid. This doesn't always happen. Sometimes the expected increase in drops of NaOH with vigorous exercise does not occur.
Several things can happen.
As exercise rate increases, ventilation also increases, which can actually flush more CO 2 during vigorous exercise than during moderate exercise. Also as exercise becomes more vigorous, the muscles switch to anerobic respiration producing lactic acid and less CO 2 over a given time. If you do not have access to three sets of respirometers, do each procedure separately. The respirometer works by creating a vacuum through the absorption of carbon dioxide by the soda lime. The lab manual tells you to make measurements every minute.