Of course you have to qualify yourself to be able to claim SDP.
So presumably you have DLA/PIP yourself, as well as an Income Related benefit that the SDP can be added to?
The governments own leaflet on disability premiums
https://www.gov.uk/disability-premiu...rt/eligibility
If you can tell us exactly what benefits both you and your partner are currently getting then someone will give futher advice.
Or you could try one of the online benefit calculators (turn2us or entitledto) to check what you should be receiving:
https://www.gov.uk/benefits-calculators