The source is even in the headline: Government Accountability Office
https://www.gao.gov/assets/700/698693.pdf
Now, that was painless, wasn't it. Here we go.
Have you ever worked on a new weapon system? I have. Spare Parts are sometimes hard to get. And those parts on a weapon system that is in thee field and being upgraded at the factories, you will have to have a Field Level Maintenance support to modify many of those parts to fit the ones that came out before the modifications. We went through that with our F-15A models after they were producing the F-15C. The parts were for the C and we had modified A to C models. Not just the big stuff but the little stuff as well. They should have given the updated A model F-35s a different letter disignator if they were true to form. It's about 35% the same aircraft as the original As that were first accepted.
Keep that 35% figure in mind. The original idea was for all three to share at least 75% of the parts. They only share about 35% of the parts between the 3 of the models. Trying to have one supply chain for all 3 just doesn't work. They tried some new ideas. As usual, they are going to have to do a rethink. It's like that with all developing weapons systems as complicated as the F-35. And then you add in the 3 variants it gets even more difficult.
Not to worry your pretty head, the ones that are sitting just off the possible front lines have the parts and have the Field Level Maintenance to go with it. While the overall fleet may be operating at between 27 and 35%, the Marine F-35Bs flying off the small carriers are at a 70% while the Air Force F-35As in Asia are above 75%. The ones in the States or in non hot areas will just have to put up with it until they get a handle on it.
Funny, I also saw the same problem with the Antique Aircraft where we had to make parts, learn to relearn about their modifications that weren't even on the books. Yet we still met our missions. Push comes to shove, all the Branches of the US Military can make it work if called upon.
It's the People that maintain the system and fly the system that makes it all work, not necessarily the weapon system.