This problem of the carabiner sometimes not releasing on descent is not a design or slot angle issue. If it was, everyone would experience it, but of all the HHs out in the field it seems only a few have this problem. From the experimenting I have done, it appears to be a surface texture issue. When two hardened metal surfaces are subjected to high- pressured loads, any small imperfections and milling marks can act like the gripping teeth on an ascender. This same phenomena can be seen on trigger mechanisms and is the reason careful polishing is used to reduce load needed for trigger release.
For anyone having this problem, I have found that taking some time and working the contact surfaces of the HH and possibly even the carabiner with progressively finer wet/dry sandpaper, ending with 400 grit, until polished smooth will solve this problem. Remember you are not trying to remove material, you are just polishing it.