The authentic aspire nautilus bvc replacement heads are only available in 1.6Ω & 1.8Ω.
The nautilus X replacement heads are available in 1.5Ω & 1.8Ω.
Are you using a standard mini or full size nautilus or the newer nautilus X ?
What power level /settings are you using?
The R40 does have an adjustable initial resistance feature (+/- .05) in TC / TCR modes, SS, Ni, Ti.
The nautilus replacement heads should only be used in watt/power mode.