ecso, a follow-up to my previous reply.
I got curious a while back about the physics of melodicas. I did a lot of research into the topic then, and I recalled that there are a number of factors that affect how responsive a reed is.
Assuming the reeds on the 44H and 44HP are the same, then the thickness of the reed plate and the shape of the hole through which the tongue travels can affect the responsiveness. As I recall, the less thickness, the more responsive the reed is with less air, but it will fail to perform loud notes properly. More thickness and the louder notes work, but the quieter notes will not sound.
I don’t know about melodicas, but on some other reed instruments the sides of the hole have a slight taper (about 5 degrees). Supposedly, this provides the benefits of a thicker hole and still allows the quieter notes to work.
If the reeds are different, then there are some other options, although it’s the same sort of trade-off. More flexible reeds perform well quietly, but not loudly. Stiffer reeds have the opposite effect. Accordion reeds have a taper that may help with the responsiveness. Of course, accordion reeds are intended for more air than is easy to provide with lungs. People who put accordion reeds on their melodicas tend to report that they get a more solid sound but that they take a lot of air to play.
Anyway, one of these things might explain the performance difference between your hybrid 44H/44HP and the 44Hv2.