You might try creating a simple gcode file from scratch which has repeating paragraphs of this. Send it via OctoPrint and you should hear the fan ramping up/down between off and about half-way. Assuming that works, then you might look at the Settings -> Gcode scripts for content, as well as what your slicer is introducing. Finally, you might conclude that something interesting/unexpected is going on with your firmware.
I'm experiencing a similar problem. I have a fresh install of Marlin on a known good ramps. When connected via USB to my PC using Repetier host Fans cycle on and off during a print as they should. when i upload a file to Octoprint and print USB from there, same gcode, no fans. I bought the Pi 4 with the Canakit power supply first of Jan '20. No low voltage errors. Commands to terminal do enable disable fans during the print. Fan controls in UI toggle fans during print. verified gcode has fan start code.
im having the same problem with Anycubic Chiron (stock from factory) and newest Octoprint (1.7.0) connected via USB.
I can also send commands before starting a print and it works but when printing fan wont turn on. Also tried same g-code with SD card and fan works as expected, just when printing with Octoprint it does not.
Fan control also dos not work any more after a print was started and stopped before finishing.
Im not sure if fan control starts working again if print job successfully finishes (without fan they didn't for me and i had to stop them).
The "ok" feedback on fan g-codes also shows, but fan does not turn on.
when interfacing the Anycubic Chiron directly on its build in touch screen one can check the speeds in the tools tab and it is displayed correctly (like fan = 50%) but fan is not spinning when printing from Octoprint. When printer is idle the printers fan control works as expected and the fan turns on when set to any speed correctly. When printing from SD one can also change fan speed mid print to desired value, just not when doing so from Octoprint.
Im probably going to flash a new firmware on to my Chiron and try again but it is still a wired problem.
Would be happy to hear if anybody found a solution that is not just use an SD-card.
I realy like octoprint and want to continue using it.
For OctoPrint 1.7.0 was not out 2 years ago, it can't be quite the same problem.
Please open a new thread in Get Help with all the requested information that are asked for in the template.
Quite important is the serial.log. You have to enable it to get a proper log.