Refusal to calibrate suggests that handheld calibration may not have been correctly carried out recently. 

A full calibration (low then high) should therefore be carried out using PC software. 

If you do not have the PC software, it is available for download at http://www.analoxsensortechnology.com/submersibles-c11/submersibles-c12/sub-aspida-p30 then click <Downloads> and <Aspida - PC Tool & configuration manual>.

I recommend carrying out a full PC calibration ensuring advanced settings is selected and local atmospheric pressure is correctly entered.