Yes, the F-35B, the STOVL variant has been placed on probation, but the entire program is encountering difficulties.

Most recently it has been revealed that there are problems with the aerodynamics and propulsion for all the models:

Flight testing so far has revealed problems with handling in the transonic and medium angle-of-attack regimes, [reportedly problems with wing drop/sideslip akin to those discovered in flight test for the F/A-18 E/F] and a problem with screech – destructive high-frequency combustion instability in the F135 afterburner – which is preventing the aircraft from achieving maximum power. [Which produces shockwaves, think the “snapping” on a an improperly lit Bunsen burner, writ large]

You know, that alternate engine for the F-35 is starting to look a lot better, huh.

Additionally, there are very real continuing problems with software, with the advanced helmet mounted displays and the vaunted 360° sensor fusiuon for the pilot not working properly.

As to the B model, weight problems is literally a killer.

With conventional and carrier launched variants, weight gain cuts into performance, payload, and range, but with STOVL aircraft, it means that they are simply unable to perform their basic functions:

Gates said that solving the unspecified technical issues now afflicting the aircraft “could” add cost and weight; the program office says that it “will”, and that it will take two years to “engineer solutions … and assess their impact.”

………

Vertical landing is a nonvariable requirement. The required airspeed is zero and can’t be adjusted by a few knots to compensate for extra weight. The JSF key performance parameter for bring-back load – corresponding to two 1,000-pound JDAMs and two Amraams – was set early on at a minimal level.

One reason that Lockheed Martin’s shaft-driven lift fan (SDLF) concept was a winner in 1996 and 2001 was that it seemed to offer thrust margin for vertical landing. At the start of SDD, the F-35B was projected to have an empty weight of 29,700 pounds – not a bad place to be in with (then) almost 40,000 pounds of vertical thrust. But, in the weight crisis of 2004, engineers found that the jet had ballooned to a far higher figure (never actually published) at which it could not land vertically with normal fuel reserves, let alone weapons.

………

Still, the redesign left a fundamental problem: the bring-back load (around 3,000 pounds) was only 8 percent of the landing weight. The result is that the F-35B couldn’t tolerate any OEW growth or thrust shortfall. The engine and transmission are maxed: that’s the issue underlying the repeated delays in powered-lift testing, chronicled here and here.

This is a mess, and much of it is driven by unrealistic requirements for commonality, and placing too much emphasis on the STOVL version, which wastes weight on the A and C variants.

When I worked on the now FCS, it was clear to everyone working on the program that the commonality requirements made the systems more expensive and less combat effective, and the same thing is happening with the F-35.