This brief produced by Lockheed Martin and the F-35 Joint Program Office highlights the trial and error process of the jet's durability testing. A lot of the problems identified are particular to the US Marine Corps' F-35B, which is perhaps not unexpected. But the other versions have some issues too.
None of the problems are what engineers might call "showstoppers". But I am reminded of the words of former Joint Strike Fighter (JSF) program manager Rear Admiral Steve Enewold (now retired and leading the MQ-4C program at Northrop Grumman), who led the F-35 SWAT (STOVL weight attack team) effort to cut the jet's weight by some 3000lbs almost seven years ago. Enewold said at the time that he worried that the program was inadvertently reducing some of the F-35's "good weight." That's not necessarily what's causing the issues here, but it's something to perhaps keep in mind. (Read Air & Space's article on the SWAT effort here from 2006)
Nonetheless, it must be noted that the point of these durability tests are to find problems and then fix them.
Meanwhile, the JPO and the industry team have figured out what caused a fueldraulic line to fail on an Eglin-based F-35B on the 16th of this month. No design flaw or anything, just a manufacturing defect.