Question PSU doesn't turn on when shorting PS_ON to ground once, only turns on when i short it multiple times

[apsonus]

I have quite a peculiar problem.

I tried building a PC for someone using mostly parts I had laying around that I wasn't using because of a very limited budget.
The motherboard (IH110CX) I'm using for it was taken from a Lenovo PC, and instead of the standard 20/24pin ATX connector it has a 12v only 10pin connector.
So I resoldered the needed wires from the 24pin connector I had from a standard ATX PSU to the 10pin connector that fits in the motherboard, however soldering the two green PS_ON wires together didn't work (as in the PSU wouldn't turn on at all), so I soldered the green PS_ON wire from the PSU to a switch and to GND.
Now it all does work flawlessly once I manage to get it running because I've already tested it multiple times, but getting the PSU to start is the actual problem.

When I short PS_ON to GND (both through the switch and directly through the wires) the fan only spins for like half a second, I also have a LED hooked up to one of the 3.3v wires from the PSU that turns on when the PSU is on so it's not like the fan only spins at a certain temperature.
It's also not like it only turns on on load because I've tried both on itself and with the components connected to the PSU.
It ONLY turns on when I either spam the switch on and off or when i short the PS_ON and GND wires and let go and then short them again a few times.
I've also noticed I have more luck getting it to start when I ground PS_ON to something else that's grounded rather than the actual ground cables from the PSU.

I've tried looking for a solution everywhere and I couldn't find anything, I've only seen people saying "it's a faulty PSU" but like I've said it works perfectly once it turns on, all the voltages are correct, all the cables work, everything. I've tested it in multiple ways including running it for more than a day under full load and nothing ever went wrong so it seems like it's fine and stable. It's just this one thing that's the issue. Thanks for any help in advance.

The PSU is a Modecom Feel 500, I got it from an old PC I wasn't using anymore and it worked perfectly fine in there.

 

[Lutfij]

Welcome to the forums, newcomer!

First,
Modecom Feel 500
Using Google image search, that unit does not belong in a PC. More so in a trash bin.
I got it from an old PC
How old is old?

Second,
The motherboard (IH110CX) I'm using for it was taken from a Lenovo PC, and instead of the standard 20/24pin ATX connector it has a 12v only 10pin connector.
You should've looked into an adapter/harness that adapts a 24pin connector to the 10pin found on the proprietary motherboard or the right PSU for the motherboard at the very least.

it worked perfectly fine in there.
What are the specs to the old build? What are the specs to the build with recycled parts?

 

[apsonus]

Welcome to the forums, newcomer!

First,
Modecom Feel 500
Using Google image search, that unit does not belong in a PC. More so in a trash bin.
I got it from an old PC
How old is old?

Second,
The motherboard (IH110CX) I'm using for it was taken from a Lenovo PC, and instead of the standard 20/24pin ATX connector it has a 12v only 10pin connector.
You should've looked into an adapter/harness that adapts a 24pin connector to the 10pin found on the proprietary motherboard or the right PSU for the motherboard at the very least.

it worked perfectly fine in there.
What are the specs to the old build? What are the specs to the build with recycled parts?

Why exactly does it belong in the bin?
It is quite old I'd say like 7 years at least but it was barely used.
I did look into those adapters but the thing with them is they work pretty much in the same way, they short PS_ON to GND to have the power supply on the entire time but I wanted to have it controlled through a switch.
I don't remember the exact specs of the old build and I don't have the parts with me anymore as I gave them to a friend but I remember they were around 9 years old, PSU was switched later.
The specs of the new build are ih110cx, i5-6400, 2x8GB ddr4 2133mhz, radeon rx 580 4gb. I know it's bad but it's good enough for what it's going to be used for.
I don't think the specs have anything to do with it as the PSU doesn't turn on normally on it's own with them disconnected.