and if replacing the fans, the fan connectors on the motherboard have very gentle pins that tend to bend very easily if the fan is inserted in wrong angle or using brute force.
or have you already opened to machine for dust cleaning etc and then when you removed the fans from their connectors you accidentaly bent the pins, you should check if this is the case.
I've seen people buying new fans and those "are broken from the beginning" when the problem is simple, bent pins.