In order to lock jets, first you have to start locking to them when there are coming to your directions, if he got ecm, he should it when he is being locked, So your first one will be missed and shoot the second one right after when he got nothing to defend against your rocket and when he is going away from you and still in the range of igla/stinger ( but you need to be quick in the 2nd one).
If he is noob pilot and he doesnt use ecm ahead of time means ( he already got locked and shot on him and then he uses his ecm ) You will get sometimes hit on him.. ( ECM sometimes works and sometimes doesn't work in this situations)
To use ECM, you need to use it 1st second when you are being locked.. do not wait till they launch their stinger or igla