The combo box has signals which need to be connected somewhere. The signals which could interest you are activated() and currentIndexChanged(). Depending on the relation of the combo box to the rest of your UI, there could be some default connecting but it need not suit you. This might be the reason why the combo box reacts sometimes and it does not do other times.

Create signal handlers for the combo box, connect and place breaks in the handlers. Debug and see whether your handlers are called and whether the handlers do what they should do.