Circadin / Melatonin not working

hi. I wonder if anyone has found the 2mg Circadin doesn't have an effect? Paediatrician said we can double the dosage to 4mg but that doesn’t work. Lovely daughter is still up and down stressing about getting to sleep. When we doubled the dosage the Paediatrician said sometimes Melatonin doesn’t work and if that’s the case they may withdraw it !!! I’m horrified at this, my daughter needs to get to sleep but what am I to do if the medicine isn’t working ? 

Juat wonder if anyone else has experience of circadian 4mg not working ? How did doctors deal with this situation ? 

thanks in advance x