Alone, this problem with Iran's stockpile of heavy water may not be a big deal:
The International Atomic Energy Agency (IAEA), which is policing the deal, said Iran's overstepping of the limit on its stock of a sensitive material for the second time this year risked undermining countries' support for the agreement. [emphasis added]
But the highlighted portion highlights a problem with America's ability to enforce the deal. America doesn't enforce the deal, the IAEA does and we don't control the IAEA.
And Iran is basking in the glory of disputing what the meaning of the word
"Where is (the) limit?" Iran's envoy to the IAEA, Reza Najafi, told reporters on the sidelines of the board meeting, adding that the country was preparing to export more than the 5 tonnes of heavy water it originally informed the IAEA of.
"The JCPOA is very clear," he added. "It says that the needs of Iran are estimated (to be) 130 tonnes. Who is the native English speaker to tell me what estimated means?"
Yeah, this will work out great.