According to USIP, Iran made some demands that IAEA drop [all] additional investigations, which didn't sit well with the West:
The primary challenge was on the political side as of early 2023. The world’s six major powers – Britain, China, France, Germany, Russia and the United States – were very close to reaching an agreement with Iran in August 2022. But the talks stalled after Iranian negotiators made unrealistic demands beyond the scope of the JCPOA. They stipulated that Iran would agree to a deal only if the U.N. nuclear watchdog closed an investigation into traces of uranium found at three undeclared sites within a specified period. They also demanded that the IAEA never again investigate Iran’s past nuclear activities.
FWTW, the IAEA leadership wasn't too willing to go along.
IAEA chief Rafael Grossi told CNN [...] that the agency would “absolutely not” close its probes, noting that “so far Iran has not given us the technically credible explanations we need to explain origin of many traces of uranium, the presence of equipment at places.”
OTOH the same source mentions that Iran officially had dropped some demands in August, namely that IRGC related entities be unsanctioned. But another source mentions that (despite that) Iran hardliners were betting that a hard winter in Europe coupled with lack of Russian gas would make Europe more amenable to Iran's heightened demands.
Marandi, who is in the inner circle of Iran’s hardliners, argued last year that the war in Ukraine and the cut in Russian energy supplies would leave the Europeans in a winter freeze and Tehran would get strong leverage on the nuclear issue.
But that didn't really happen. And probably based on the same ideas (that the West will fold), they supplied drones to Russia at around the same time, although Iran still claims that had happened before the war. And now the US [consequently] says it has been "played" and that despite Iran's renewed affirmation in the deal, it's not really interested.
“What’s the point?” [US Special Envoy for Iran Robert] Malley said about the talks. “Why should we focus on it if Iran comes back with demands that are unacceptable? At this point we’re not going to focus on the nuclear deal because we can’t sort of keep going back and then being played.”
Vice-versa, while the Biden administration might not have mentioned publicly [AFAICT], some [Republican] hardliners in the US were also publicly betting (at least in the winter) that protests in Iran would [instead] weaken the regime.
The E3 essentially acknowledged the impasse
“As we have stressed many times, there are currently no negotiations on the JCPOA,” [German Foreign Ministry spokesman] Christian Wagner told reporters in Berlin.
And that was despite the IAEA chief's visit to Teheran, which did yield a minor agreement on more inspections at some sites, but otherwise seems to have deferred most other outstanding issues.
The mutual seizures of oil tankers in April this year (which had some precedents in 2019 etc.) probably didn't help either, even if there's been no official tit-for-tat claimed this time, AFAIK.
Additionally, some commentators have suggested that Iran is even less interested in making concessions on a nuclear deal now that its relations with Saudi Arabia have improved a bit on a mutual basis (enabled by China).
I.e. mutual trust is at or close to another nadir. And almost everyone involved seems to have played some kind of waiting game since last summer, hoping to negotiate from a better position.