The fact of the matter is that it is difficult to shoot down a modern jet - far more difficult than it was even in Gulf War 1. Over the past two weeks we've seen several close calls, and today they shot down an F-15E Strike Eagle and took out an A-10 Warthog, the latter being the easier to do of the two.
Sure, one could argue that it's just luck, but as it turns out, the Iranians are human beings, and human beings are above all learners: they are figuring out tactics and capabilities. You can shoot down jets in theory and you can have cracker jack technology, but until you actually have to do so in anger, it's all theoretical. But now they're developing praxis.
Trump thought the war would be over in a week. His problem now is that the worst is yet to come. They're not going to get WORSE at shooting down jets. They're going to get better at it. They're not going to get worse at shooting down SAR helicopters. They're going to get better at it. They're not going to get worse at launching missiles and targeting. They're going to get better at it.
That's where we are right now. They've figured out something about US tactics and their own capabilities that is allowing them to get close and now hit jets with better accuracy.
This is the first day of the rest of the war.