You could go to olive oil next, and there are other, stronger techniques. But sometimes they just don't come clean, short of something like electrolysis - but that ruins them completely in my opinion. You want patina, and even dirt is OK to an extent, as in coins that have "desert patina," just not so much that obscures the details.
Yeah I've tried olive oil before and don't really like the dark patina it gives the coins sometimes. And with electrolysis I actually get decent results doing short periods of 10-20 seconds although the patina usually does take a bit of a hit.