I was expecting this to be a Honeywell 4073 mid position valve before I
saw your photo. It still 'looks' like one but clearly isn't.

Generally, the motorised valves with a replaceable actuator have a
dimple on the top of the casing. it is not clear from yours whether you
have this or not.

Anyway, the problem is one of two things; mechanical or electrical. In
order to diagnose the fault you will need a multimeter and will have to
poke around in the wiring centre. An electrical neon screwdriver will
not be sufficient to do this.

Are you confident of not electrocuting yourself?