It could be because the CYA is low. Wait a week after you last added CYA or used dichlor or trichlor and then run the CYA test again. If you are still between 30-40, you'll probably want to bump it up a little to around 50ppm and see how that works for you. Just to make sure you realize that your chlorine level will not stay constant even with adequate CYA. It is normal to lose some every day.