which is meant to be the best powder on the market
Says who? I haven't used Ariel in yonks.
Washing whites with liquids just doesn't work. Yes, they're clean, but they lose their colour and without oxygen based bleaching agents, towels, for example, retain a lot of the grease and oil that comes from skin. Speaking from experience, before we lived together, my other half was an exclusive liquid user and ALL his white towels had gone grey. They were the first thing to go, replaced with lovely new white towels and a box of Persil Bio.
I also wouldn't ever wash my whites on 40 degrees - they need a hotter wash. Being perfectly honest, I do think that whites on 40 with liquid is pretty manky.
I've also found powder to dissolve a lot better than liquid, which is more dense than water and tends to gloop up in the machine and leave more residue around the tub. If powders are leaving residue, you're either using the wrong cycles or your machine isn't rinsing properly.