power-on failure issue that looks like timed

This is something for a GENIUS to resolve!

I have tried all suggestions i could find online. The techie took the lappy apart several times as well.

Problem still a complete mystery!

The problem: One fine day i wanted to power-on my lenovo Fex 2 - 15 .. but it did not want to come on :-(

I tried a bunch of times and it finally booted up and work as flawlessly as ever before.

Next time i wanted to power it on same theatre, just that it took a bit longer of trying until it finally powered up.

Eventually it began to look like it simply will not power-on after a one-time use, usually for about 24 to 48 hours!

We checked all hardware inside, nothing looks defect. We changed the flat round CMOS battery for a new one, we checked if we can power it up without the battery being connected to the motherboard (or where ever it is connected to), and in addition i tried all these more or less amusing tips like static power drain, press power button for 5 million seconds once, then press again just for 1/10th of a second, or remove the left wheel from your car, now try to power up your laptop again, you name it, no change, the laptop only powers on if it wants to, usually after a minimum of 24 hours of being turned off.

Battery charges just fine, but one of my guesses is that it may no longer power up if the battery is fully charged to 100% and needs to lose a bit of charge, say down to 95 or 93% to be willing to power-on (just a guess)?

I still have Win 8.1 in use, works absolutely flawless once it has powered on, but, as ever so often, there is always the guess if maybe a Windows update screwed up something?

The final guess .. maybe there is some invisible issue with the motherboard, but if there was, wouldn’t this impact the general performance of the laptop with problems also once it’s fully booted (which it does not)??

I have also tried a few help forums, but absolutely zero answers that resolved this very strange problem.