Kids flossing

Ok so my 6 year olds teeth are so hard to floss like it's hard to get the flosser in his molars it doesn't go in easy like with his sisters does anyone know why this is ? And sometimes because of hard it is to get the flosser in he bleeds a little sometimes and sometimes no

He barely went to the dentist like a month ago and she didn't say nothing concerning about his gums or anything.