How do your report IRS fraud?

First of all I know about Form 3949-A, but so far the IRS has just ignored my report (I guess they get too many of these or maybe the IRS has bigger fish to fry). But I know a couple who have been committing fraud for the last 5 years. They file "married filing joint" every year but they only report the wife's income. The husband who makes more money than his wife ,doesn't include his income on their tax returns. As a result they get money back from EIC and WPC, plus all her money back on her W-2. I am absolutely positive about all this and have given the IRS their names, addresses, as well as the place where her husband has been working full time for 5 years. For some reason the husband is paid with checks every week but his employer who is a self-employed race horse trainer, doesn't deduct taxes. Does the IRS not really care about this?