Did you really fixed the 12V high current wires with a zip-tie to the shunt? That's a very bad idea. Especially in a wooden box. Use the right material or you might start a fire! And the soldering in the first picture won't do either!
Edit:
The longer I look to the pictures the worse it gets.
1. Are the PSUs rated to run in parallel with out a resistor to prevent unwanted currents because the PSU modules are not 100% identical?
2. The color-coating on the shunt might changed the value of the shunt.
3. The uninsulated crimps on the high voltage side don't look very trustworthy
4. I'm not sure if it is a good idea to use uninsulated high voltage parts to a silver coated surface (hint these colours might contain metal flakes)? And yes I saw the useless plastic washer (the screw renders it useless) and the distance of the contacts to the surface is to small.
1. I don't need criticism on the project. I need help with the issue
2. The PSUs are 100% identical
3.Just because you do like the look of something doesn't mean it's not functional.
4. The only thing that's acting as a plastic washer is for the output that's on the outside of the box.
5. Again I need help with the issue not a negative OCD comment
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Regardless of if you like what twoflower said or not, they're all valid points.
That wiring and box is a mess.
Your power supplies look like they're (HP?) server PSUs that have been stripped out of their cases, they may be OK to parallel, difficult to know unless we know where they're from though.
If they are server power supplies, why did you strip them out of their cases? Those cases are ideal for EMC shielding (which is one possible cause of your problems) and prevent 'random' stuff falling into the PSUs which makes them a lot safer, plus they contain the shrapnel when they go bang (which they can do, sometimes quite spectacularly)
To parallel power supplies they have to have been designed to be paralleled so unless that's explicitly specified in the data for those supplies then you have got potential problems (pun intended), it might even be the cause of your ammeter jumping around as they both try to set their output voltage to what they think is correct, try disconnecting one and see how the ammeter behaves then.
Was your ammeter specified to work with that shunt? How are you powering the ammeter?
Can you try moving the shunt sense wires (the thin ones) away from other load carrying wires, including mains wires?
You're probably dealing with millivolt signal levels and if they're close to wires carrying current then it's possible that's causing problems with 'pickup'
Load balancing resistors are wasteful but useful, you add a low value resistor in series with the output of each PSU, the resistor 'soaks up' and wastes the voltage difference between the outputs as heat but that poses another problem, you have to get rid of the heat somehow.
Spraying everything silver is a bad idea, makes it easy to make a mistake when you're fault finding and at best lead yourself up the wrong path, at worst you end up with a pile of scrap components, as others have pointed out, metallic paints often have metallic flakes in them, bit of a giveaway in the name really.
Uninsulated mains connections are a *really* bad idea, it's a moment's work to short them out with a loose screw, screwdriver, small rodent, your finger or worse, someone else's fingers, insulate them properly and not with the black snot you've used for the 12V outputs.
The current shunt *really* needs to have crimped and bolted connections, again others have said that but again, millivolt signals and you've got *really* poor connections on a high current component, at best it won't work properly, at worst it'll arc and burn up the wiring.