I agree that a complaint from a parent is taken more seriously than a complaint from another member of staff. It may be that the member of staff is frustrated with nothing being done and hopes if you complain, something may be done.
I've advised this once to a parent (just the once in a 20 year career) about a member of staff who was appalling and the situation was being ignored. I was middle management at the time. I won't go into details and I can assure you, it wasn't my first choice in how to deal with the situation but something had to be done. It worked btw.
My DD has ADHD and ODD currently last year of MS. Yes over the years she hasn't got on with a lot of teachers as they haven't been able to manage her for one reason or another. Anyway she's having problems with another teacher. Staff have told this teacher how DD ticks. She doesn't process info very well and prefers back up step by step instructions to help her learn and retain.
This teacher wouldn't help her or explain again or anything as in his eyes she wasn't listening, she told him it seems to go in one ear and out the other and can't help how she is. As he refused to help she has sat doing nothing. At the end she has went to see the senco who I think is the teacher who has told me to complain.
I just feel, here we go again and can it not be sorted in house like they have in the past, why make it more official?