I can confirm Delta is very reluctant to pay EC261 claims, even if valid. I had to involve the Spanish regulator before they paid a valid claim I had for a flight ex-BCN. Smart business decision - few customers would bother to go through the hassle I did for 600-800€, and this stinginess probably saves them a ton of money. Imho, it’s a flaw in the regulation; if the airline denies your claim and it is later confirmed valid, you should be entitled to treble damages to discourage this sort of behavior.