I can only guess at the reasons for the MO deciding not to act. One factor could well be that for a criminal conviction it is necessary to prove that the failure to declare was deliberate. Another factor could be that the MO felt that it just wasn't worth the expenditure of time, given that no benefit would flow to the MO's council even if a conviction was obtained. And the outcome might not be very satisfying - Councillor Spencer Flower the first person to be prosecuted under the legislation. He was convicted, but was given a six month conditional discharge and allowed to remain in office. Bringing a private prosecution would be expensive (and I suspect may be vulnerable to being blocked by the CPS) and you might well feel it not worthwhile spending money for that kind of result.
But if you feel it worth further consideration, I think you will really need professional legal advice.
Personally, I feel that the current regime is ineffective, and that it would have been better to have carried out a less disruptive reform of the previous system, using the Standards Board.