Won't let me target blue monsters?

I was trying to do the forest lord spawn today with my mage. I noticed an interesting thing.

If I used target cycling macros. Target nearest -> cast spell -> current target I was able to attack the pixies without any problem.

If I tried to use a chain lightning or something, when I targeted the pixie, I got the gump asking me if I wanted to flag. I say yes... and nothing happens.

I tried targeting the ground near the pixies, and it seems to do no damage to pixie. If I target the...

Won't let me target blue monsters?