Maybe they've been taking down jets the entire time and you've simply been lied to and the situation on the ground is different than what you believe.

CENTCOM tweeted that no shootdown occurred only for Iran to show the wreckage and one of the ejection seats. CENTCOM said Iran didn't shoot down the F-35, but it apparently crashed in Saudi Arabia and we sent out a Chinook to run search patterns to find it.

CENTCOM claimed a single Kuwait (ghost of Kuwait?) shot down three different F-15s by accident, but those planes were close enough to Iran that they could have been targeted by Iran (which seems more likely than the massive chain of mistakes required for the Kuwaiti shootdown to happen).

CENTCOM also only talks about US planes shot down and excludes Israeli F-15 that have been hit. CENTCOM also doesn't count all the very expensive drones that have been shot down, but their total value is at least a half-billion dollars.

Finally, CENTCOM is straight-up lying about air superiority. They claimed they'd switched to gravity bombs, but people instantly noticed that all the planes going up were using long-range JASSM stand-off missiles so they don't have to actually go very far inside Iran and can keep their planes in a safer section of the country. CENTCOM loaded up a B-52 with JDAMs and took a couple pictures, but all the pictures after that photo-op still show super-expensive JASSM (costing as much as $1.6M each).

If our planes never enter their airspace, there's nothing for them to shoot down. I'd note they've also shot down a couple of JASSM missiles which is interesting itself as the radar cross-section of a JASSM is believed to be pretty close to an F-35.

Iran is currently using bongo trucks with an IRST and a couple missiles that can even loiter in the air if there's no target. Being electro-optically guided means they are passive and the human element makes them a lot less likely to miss due to chaff.

These little trucks can hide ANYWHERE and because they aren't a massive multi-vehicle setup like a Patriot or S-300, they should be able to relocate often and quickly (they might even be able to stay mobile while in operation). This mobility combined, ability to hide as a normal truck, and completely passive sensors make them almost impossible to find and destroy.