Trouble using ACM to lock targets

[ALF13]

I'm having trouble getting locks using the ACM modes with Aim 120s. It works fine most of the time but other times it doesn't lock anything. I'm trying to figure out what I'm doing wrong

I know that I have to be within 10 miles but sometimes I have to get within 3 before the radar acquires the target. It happens with all the modes. 

What could be causing this? 

Thank you

[Archer.xd]

There are a few issues reported and already being worked on. ACM modes will struggle in lookdown conditions and/or if target has ECM on. You are not doing anything wrong, best is to wait for fix.