Problem with PWM pins on Arduino uno
Try different pins than 5 and 6, because those PWM generators share the timer with delay(), which I'd interpret as: You use analogWrite() to set the timer to fire every n microseconds to toggle your PWM signal  -- and then you call delay(1000), which sets that same timer to fire in one second to continue the program.

Pins 10 and 11 do use a different timer, so try these.

Reference: https://www.arduino.cc/reference/en/language/functions/analog-io/analogwrite/ 


  "The PWM outputs generated on pins 5 and 6 will have
  higher-than-expected duty cycles. This is because of interactions with
  the millis() and delay() functions, which share the same internal
  timer used to generate those PWM outputs."