Make sure your new cartridge is dissolving, if it is something(bather waste, risdual soap ect) may be chewing up the bromine. My suggestion would be to pull and clean your filters well and shock again. Shocking re-establishes used bromine, so after shocking you should get a reading. If you do and it does not keep a reading for 24 hours, shock again.
If you still do not have a reading, bring a sample to your dealer and have it checked. Something may have gone wrong with the test strips.