Yeah, some wools just don’t like to felt, it may be that they’re spun too tight or mishandled or have been bleached are a plethora of other silly little reasons.
The soap to detergent question is one I’ve heard so many different opinions on - but in the end the one the comes up on tops is a dish soap like dawn usually works best (though I’ve used laundry detergent and natural laundry soaps and they’ve done fine as well)
The hot-to-cold temperature change can be extremely helpful in the felting process. If you wash your item in hot water and go in every 10 minutes or so and swish your item in a basin of (really cold) water before placing it back in the washer, it can really help.
How hot your water is (hotter the better) makes a big difference too.
As far as what yarns felt well - I like how tight Patons Merino felts, but it tends to felt REALLY FAST and have a sort of boucle texture to it. Knitting the item-to-be-felted loosely will also help. I like knitting felted items with wool reclaimed from wool sweaters but there’s always a bit of ‘wait and see’ when i do that if I don’t swatch first.
I’ve generally found that the looser the twist of the yarn the tighter it tends to felt. But this may not always be the case.