Double Check valve issue

Hi Guys one for our plumbers perhaps Tom ? my daughter had a leak from the filling loop on her boiler, dad went round to have a look and the leak was coming from the valve spindle (bollofix) so nothing to tighten, I bought a new filling loop with double check valve attached, on fitting this I found that the system would not accept water (yes the double check was on the right way around) I took the valve off and tried a new bollofix which was how it was in the first place and it topped up straight away, I have left it like this for now, but any idea why I couldn’t top the system up with the double check valve attached ? many thanks in advance. I’ll upload photo of the loop & valve I bought from SF