my ph level keeps dropping

im doing a dwc with a top feed u know the ring around the plant and i set my ph between 5.5 and 6 i went back a few days later and my levels is back a 4 im trying to figuar out why it keeps dropping is it because the water drains thru the clay pebbles hlp me out riu

Fluctuations that go down are not good. Usually nute mixes always rise. Without knowing more i would have to say it sounds like an equipment issue. Inaccurate/slow ph monitor, bad poly in your tubing/drippers, bad hydroton. who knows. If you think its your hydroton put some ph'd solution in a cup then add some hydroton to the cup and see if the ph drops