CBOD

So I'm having a really bizarre problem. Our CBOD blanks have been consistently failing at 0.5 or 0.6mg/L depletion. But our bod blanks are passing, no more than 0.2 mg/L. This started maybe 4 weeks ago. These are always prepped at the same time, same dilution water. Obviously, the only difference is the addition of the inhibitor to the CBOD. The GGA and KHP QC'S are all within 10% of the true values. I find it hard to believe that the inhibitor which we have been using for quite a few months is the issue. Any ideas?