I2C Gnd Problem external power

Howdy! I'm facing the following problem. I have 4 arduino uno (slave) and one arduino mega (master) and a lcd Nextion. Plus multiple sensors and LEDs. I send data both in the NEXTION sense to the master and to the slave, and in the opposite direction from slave to nextion. Arduino is powered from 12V with common ground both on the power side (the power supply of each is connected in parallel) and a common ground from the GND pin of each for i2c communication making the common bridge all 5 GND. The problem is as follows: if all 5 are powered by the 12v external source, the code does not work, but if on an arduino (either master or slave) they touch the USB plug connected to the laptop of an arduino plug
(only Ground must be given as coming from the laptop) then it works. What I've noticed if I put a multimeter set on continuity between the grid from the USB plug on my laptop and the one from the socket of one of the arduino, the bipait is not continuous. It is in the form of a signal that coincides with the Arduino TX LED that turns off and lights up. If I put a multimeter between the power supply jack and the arduino usb plug, then the beep is continuous. I want to make the circuit go without the usb from the laptop. I just want to go from source power without giving it GND from my laptop. When I get GND from my laptop, the code stops. When I give her GDN from my laptop, it starts. All the gnds are checked and the connections are good. It receives GND from the source at any point. The idea is that the laptop looks like a reference signal to say what I got. Please help. Thanks a lot!

Post a wiring diagram of your setup. I'm sure there's a problem in that wiring but I'm not sure how you actually wired that setup from your description.