We have a fairly large hispanic community yet the local spanish congregation has not grown much in the last 20 years.
Different zones of the US are very different.
In the area where I grew up (Great Lakes - Midwest) there were a pretty big number of Hispanic congregations, but little growth in them. But in that area, there was very little immigration.
Where I am now (Mid - Atlantic Southeast) there are (or least were, until the economic meltdown) literally hundreds of Hispanics moving in every week. Here is where there is big growth - not only from JWs moving in from their country of origin, but also dirt-poor non-JW immigrants jumping at the chance to join a ready-made community filled with people with similar backgrounds.
It's not so much the ethnic group as it is the number of immigrants. Most immigrants to the US are Hispanic, so in the zones where they are immigrating to, there is a big jump in the number of JWs.