The meter is reading the "effective" chlorine level in the water - its not the same exact measurement. So that likely means that while you may have a sufficient chemical level of free chlorine now, you are losing chlorine at a faster than normal rate. That's an important problem to know about, so that rather than keep chasing your tail by adding more chlorine, you fix what's causing high chlorine demand. Common things to check are CYA, phosphates, and nitrates.