There are a lot of things that can go wrong and cause a compression loss... The most common obviously is the head nuts working loose, this is also the easiest to fix and requires no parts to be purchased... other than some regular nuts to get rid of the stock acorn nuts if that's the problem.
The spark plug can come loose and bleed off compression but it would have to be really loose to bleed off enough to prevent a start.. like ready to fall out loose, most the time it'll start and blow the plug out shortly after, this also leaves tell tale signs of brown to black residue on the head around the plug.
The cylinder wall can get scratched or the plating can peel off in a section which means the only fix is a new jug and set of rings... and any necessary gaskets that got disturbed in the process of removal/installation such as the head, base, intake, and exhaust. No sense trying to reuse a paper gasket, but the head gasket can usually be reused.
Head or jug warpage, usually comes this way from the factory and the engine won't start brand new, or from overheating, or a combination of not having a true surface to start with then an overheat warped it just enough to bleed off the compression. Lapping the head usually fixes this problem but it never hurts to lap the cylinder as well. I had one cylinder brand new that needed almost .040" taken off before it trued up on the lathe.
Other things can also go wrong like a piston pin clip that wasn't installed peoperly at the factory, piston installed backward, ring damage, etc that'll ruin a top end.
I always recommend the easiest fixes first like re torquing the head and checking that the plug is torqued, this'll usually revive an engine that was running but lost compression, but if it doesn't, it's time to pulll the head and look down the cylinder with the piston all the way down to see if you can see any scratches or missing plating. If this looks good and the inside cylinder walls are nice and shiny it's the rings that failed to seat before the cylinder wall glazed. Re honing the cylinder will usually breathe new life into it if you have a 48mm flex hone with the nikasil compatible stones, but can be done with a small engine hone and fine stones. The cross hatch pattern on some new cylinders is almost non existant so these can fail this way. Honing will put enough of a cross hatch pattern on the cylinder wall to finish seating the rings if they failed to seat or new rings can be used as cheap insurance... here are a few examples of these cylinders brand new, 6 months old, and the same cylinder after re honing...
This is a brand new cylinder, never installed or run... notice there's no real cross hatch pattern for the rings to seat against... just a light un polished but smooth texture...
Here is a mildly ported cylinder that's been run in one of my bikes for about 6 months of daily use... Notice it's mirror like shine (I put my finger down in the bore to show the reflection)... this is what it should look like after the rings have seated, but if new rings are installed in this one without honing, the new rings wont seat in this cylinder but it would most likely work, it just wouldn't make full compression especially at lower rpm so torque would suffer.
And here's the same cylinder after honing it for about 60 seconds with a 48mm flex hone made for nikasil coated bores. The hone needs to be slightly larger than the bore in order to cut properly, but not too big... the 48mm flex is what they recommended for the 47mm bore and it also has fine stones for finishing, you definitely want the fine stones for something like this since you don't want to remove very much metal, just enough to break the glaze and that's all. This is all it takes to properly seat the rings and it will develope a glazed finish once the engine is broken in with maybe just a hint of visible cross hatch after break in which is the desired result as what's left after break in will be just enough to properly lube the rings and help them last a lot longer.
This may or may not be the issue here, but if someone has a persistant problem with losing compression at the rings, this could be why...
Relevant to why compression was lost here or not, hopefully it helps someone.