Dana,
Has she had any changes in her diet? Changes in her routine? I give my daughter prune juice every day to prevent constipation. I used to give it "straight up" but she got sick of it. Then I would mix half apple juice and half prune juice. She got sick of that, so now we do "prune juice soda". It is half prune juice half caffeine free cola. Sometimes I have to give it to her a little more, if she is constipated.
Some kids just do end up needing miralax every day.
I find that certain things will constipate my daughter. Traveling, vacations, change in diet, things like holiday eating. I really need to stay on top of things to prevent a problem. I make sure she also eats lots of fruit, veggies, dried fruit as well. Occasionally she will need a laxative, like miralax, or milk of mag. My daughter has had two very serious bowel obstructions, once at birth and once at 6 months old, both requiring surgery, so I am totally bowel obsessed. HTH