I'd have to see the details on the part itself. It makes no sense that a "mechanical" control doesn't work when an "electrical" control does. I'd expect the "electrical" control to operate a solenoid which then operates the same the "mechanical" control to open/close the valve.

It sounds like the (plastic?) linkage between the lever and the valve gate(s) has failed. I'd remove and inspect it (some disassembly may be required) to see if something 'looks broken' and either try a fix or replace it.