Curently regressionTrigger triggers email when THERE IS DIFFERENCE in failed tests, which doesn't cover fully regressions.
Example: build #1 testA failed, test B ok
build #2: testA ok, test B failed.
In both cases 1 failed so for build #2 regressiontrigger won't trigger email.

return build.getTestResultAction().getFailCount() >
previousBuild.getTestResultAction().getFailCount();

But in fact it is the regression because new test failed.

So modify existing RegressionTrigger or add new 'NewFailedTestsRegressionTrigger' with
public boolean trigger(AbstractBuild<?, ?> build, TaskListener listener) {
....
for (test in build.getTestResultAction().getFailedTests()){
if (test.getAge() == 1)

{ return true; }

}
return false;