Why was Halo not as effective second time?

Every treatment with Halo can be as powerful as the first.  I think you need to go back to your treating doctor.  As a proud owner of Halo, I have this suspicion:  when a Halo treatment is started, the depth and density parameters must be set prior to measuring the size of the treatment areas.  If you don't, the system defaults to a very light treatment that has no erbium component included so that it is a very quick, zero downtime event. This sounds like what happened to you.  With the right settings we always can give you the 'burn'

Ok, if you had a lot of pigment, and a heavy (high density setting of erbium laser was used) then the perceived results after the first treatment will be greater  (see video for explanation). Once most of the pigment has gone, subsequent perceived results will be less. Now once pigment has gone, the non ablative laser during the second treatment may take some time before you receive collagen stimulation. The before and afters, plus the treatment density and depth of both treatments will reveal the true answer. All the bestDr Davin Lim Laser Dermatologist