What controls the pressure in a MultiCooker?
The body of the cooker doesn't directly control the pressure - this is regulated by the floating valve on the lid. It sounds like there is something wrong with the safety pressure valve on the handle. My first thought is a bad gasket or something preventing the gasket from sealing. Inspect the gasket if you can get at it and make sure there's no damage that would prevent a seal.

Worst case scenario you could try to find a replacement lid. It sounds like the body of the cooker still works fine so it would be a shame to discard the whole thing.