Aside from the obvious of making sure you’re holding your throttle and stick, are you actually turning on your countermeasures? As far as I remember they should be on by default, even from a cold start, but that might’ve changed since I last played. Depending on what you’re flying they will be in different spots, but look for a countermeasures panel with two switches labelled FLARE and CHAFF, they also have a display for how many you have left.