Pop all the valleys you can find attached to the city - attack it with 1 scout and a junk hero then it tends to shift quicker.

Personally when I hit a city I try to minimise takeover hits to give a higher chance of heroes being in the city when I take it. I am to get it to 12 when it goes on tick then i hit it 13 more times and keep checking for comfort. Its only when something has prevented me from doing that I've had the sticking problem.