Hi!
I'm having the exact same problem after reflowing my logic board.
But I found a way to always make it boot.
- Unplug the battery.
- Unplug the magsafe power cable.
- Hold down the powerbutton for about 10 sec and continue to do that.
- While still holding the powerbutton down, insert the magsafe power cable and hold it for another 10 sec.
- Release the powerbutton after those 10 sec and make a "normal" press as if you would normally
turn on your computer.
If that doesn't work then try to remove one RAM memory and switch places and do the method that I told you.