I see what the problem is.

I measured the thickness of the G1/4" threads of the barbs :

all EK barbs : between 12.94-12.98mm
4 DD barbs : between 13.15-13.19mm

1 DD barb had the same measurements like the EK, so 12.98mm, 1 other barb was 13.03mm (so tight one).

So it explains why 4 of the DD wouldn't fit.