This complaint from Iran seems to me very legitimate. The US, UK, China, Russia, France and Germany no doubt have conflicting interests in this. Seems perfectly reasonable that Iran would expect the group of six would present them with a co-operative expectation for compliance:
He said the disagreements came to a head this week, as the Iranian negotiators have struggled to come to terms with conflicting demands from the six.
“There have been changes of position, and multiple positions,” he said. Iran was being forced to try to make a series of bilateral deals, rather than trying to negotiate with a group that had a common position, he said.
Iran says world powers are divided and backtracking in nuclear talks - LA Times