I've been playing around with the cuts, which has improved signal to background but the fit still isn't very good for the c-tag here.
The templates are here:
data
reference
I think it's just the statistical fluctuations mean the c-tag between the test and reference samples don't look particularly similar. The previous plot I had that Roberval commented on being particularly good was for the whole sample; with these new cuts the combined distribution looks similar (but not quite as good).

I've loosened the lower di-jet mass cut which seems to have allowed more b from (presumably) Z decays through, as can be seen by the higher b peak in the background than I was getting before. I thought this might mean the 3D fit (with the bc-tag added in too) might be better, because the background would be distinguishable from the ccbar and gg templates. The fit is below, marginally better but nothing to shout about.
here.

The b vs. bc templates are below. Ignore the axis labels, the c-likeness is actually the bc-likeness.
data
reference