Ok,
I'll bite.
Forget the arduinos for now..... What happens to the voltage if you turn on the power with all the motors connected at the HOME position?
If the voltage is good, then you can move forward and get things working.
If it is bad, turn off the power and disconnect one and try it with only 5.
Go through this trying one at a time to see if there is one in particular which is "killing" the supply.
Work out if there are any problems with any/some of the motors. Work from there.
Good luck.
Hope to hear back with good news!