Did the drop checker you got at eBay from joeandfamily have the correct regent?
I have received one from him and the regent doesn't turn blue or any other color besides amber/yellow. Unfortunately I ordered 3 extras at $3.50 each and none of them work.

Joe refuses to admit there is a problem with the regent. Says I am the only one with the problem, which is impossible. I got 4 bottles of regent from him.

I am now using API PH regent with 4dKH and it works correctly: blue and then green when CO2 level is correct.