Serial communication between arduino and raspberry pi works once, then stops working
"It always works if once after the code was uploaded to the arduino...."
Whenever you you reset the arduino the link between the arduino and the rasperry pi is broken momentarily thus causing the code on the raspberry pi to hang/ffreeze and not responding/ showing any valid results.

Try giving a keyboard interrupt by ( Ctrl + Z ) and restart the python script and that should work just fine!