As it turns out I think we all were under the mistaken impression that semi mode would turn on the jammer, which is incorrect. In reality semi mode works a fair bit differently than we thought.
As you approach a threat or threats the jammer will automatically switch between AIR, AAA, SAM1 (<= SA8), or SAM2 (>= SA10) based on the highest priority threat, but you need to hit - to consent the ecm to turn on. You will see this on the CMSC in the JMR window where it will change from SBY to OPR and starts flashing. It will only stay on for several seconds. You also need to hit 7 to drop chaff and/or flare which we were clear on.
It's also not strictly necessary to select or change a program (chaff, flare, release interval, etc.) and if you do you should use something besides the presets A-M, as the system will select the best preset based on the highest priority threat locking you up and changing them may mess with it's ability. To change programs you can use the 9 and 0 keys.
Also worth thinking about is that the 'highest priority' threat part can mess with things. The most obvious one I tried out is a AAA locking me up while there are also manpads in the area. You end up getting the countermeasure system selecting an all chaff program and manpads launching IR missiles which results in bad things.
So in case where you think you may be entering a situation like this it may be best to turn to manual mode and select a program that drops chaff and flare when you press 7. Bare in mind that if you do this you're also going to have to set the jammer to whatever mode is best by clicking the button to the left of the jammer window. Likewise in manual mode - will activate the jammer.
Countermeasures clarification
Moderator: RLG MGMT Team