Thats the point I was making, you want a variable flow rate (not fixed) that matches/modulates the DT, working on the premise of high heat output would narrow the DT and heat the room/house too quickly, cycles the HP, low and slow is best IMHO, let the HP do its thing on auto/unlimited pump speed.
if your house if not heating up i would look elsewhere for the main issue, I am sure others will help and offer some advice, this is a helpful forum