I have a block with a number of consecutive connect calls and noticed that the last one was consistently failing.
The last statement was connecting signal and slot with two parameters, like this (param types were not simple as in this example):
connect(objA, SIGNAL(sig1(int, char)), objB, SLOT(slot1(int, char));

When I stepped into Qt code I noticed that connection was failing because strings being compared were "slot1(int,char)" and "slot1(int, char)" – note the space between parameters. I had other connect statements with more than one parameter and spaces between them but only the last one was failing.

It started working when I removed spaces from connect statement, like this:
connect(objA, SIGNAL(sig1(int,char)), objB, SLOT(slot1(int,char));