Heater extruder not heating as expected rate

Hi guys, I'm looking for help with this error. Printer: RAT RIG V-CORE 3.1 - STANDARD KIT V1.3 I get this ,,Heater extruder not heating as expected rate'' at pretty much every print. I replaced the cartridge heater, temperature sensor, I changed the cables with new one. I replaced the included Phaetus Rapido UHF Hotend with a Phaetus Dragonfly BMO ST Hotend that I had in stock. Nothing worked so far. I'm using the included MGT4012WB fan, I print at 50% cooling fan for PLA to make sure the cooling isn't too strong. Where should I look into ? Thanks in advance !