Answer:

The chlorine demand (amount of chlorine needed) can change with a number of factors - organic load from stagnant water, bather load, rain, leaves, grass clippings, animals in the water, etc. etc. Probably one of these factors is different from last year or your pool accumulated organic load while it was closed for the winter. Keep adding chlorine until a free chlorine level is established. You can also add a couple drops of bleach to a large cup of pool water to make sure you can establish a free chlorine level in the small sample, then know you are getting there with the large sample.