The largest US airlines - United Airlines, American and Southwest - no longer expect the Boeing-737 MAX to fly before September. The industry regulator, the FAA, said that Boeing is obliged to eliminate another problem that was revealed during aircraft tests.

according to The Guardian United Airlines has canceled 1,900 flights through September 3, . Last month, an FAA spokesman signaled that the ban could be lifted at the end of July.
The FAA has now said it will only lift the flight ban when it is satisfied that the aircraft is completely safe, and that "a potential risk has recently been identified that Boeing should address."
What exactly happened, the regulator does not specify. According to Bloomberg , the new risk is not related to the MCAS system, the features of which led to two disasters , but also concerns the stabilizer repositioning system. On the simulator, it turned out that the control system, under certain conditions, introduces the aircraft into a dive, from which it is difficult to withdraw it.
Boeing is working on a new issue and will not submit the aircraft for certification until the issue is resolved. At the first certification, the safety assessment of the Boeing-737 MAX control system was actually carried out by Boeing itself .

https://twitter.com/FAANews/status/1143985330951008257

Boeing 737 MAX flight ban extended due to new problems