Hi there! Relatively new player here. I'm on BDA+ 1.5.6.0 and I've been wondering why I can't get my AIMs to lock on target. It sounds like I've stumbled across the same issue as Rakete here, whether that be user error or a bug. I'll try rolling back to 1.5.5.1 to see if that works. In the meantime, here's a post I made on r/KerbalAcademy yesterday about the same issue:
Love your mod, despite the bug! Thanks for actively maintaining it.