Pony dropping her head

[EllaBella]

We have recently bought a pony for my 8 year old daughter who has been riding for a couple of years now. I however know not a lot about ponies. Jazz is a real sweetie, she doesn't mind being groomed, having lots of children around her or having a bath, she just stands there!! But when my daughter rides her in the menage after about 20 minutes Jazz starts to pull her head down nearly pulling my daughter off. I am not sure how fit Jazz is and how much she has been ridden in the past as she is needs fattening up a little and I wondered if she gets tired.It is always when she is trotting never seems to be when she is walking. In my daughters lesson today we put some grass reins on her which didn't seem to work, not sure if they were tight enough though. We tried to persevere with my daughter pulling Jazz's head up each time but Ella is not that strong. After a few more times of her dropping her head she then refused to walk on anymore even when kicked on.
I am really worried that this is going to become a real problem and will put my daughter off riding her pony but at the moment she loves her.
My friends that own horses have said that most new ponies try it on and we should be able to overcome it, but I'm not sure what to do, also the riding instructor is full of doom and gloom and says that the pony should not be doing this. :(

[jUmPingIsLifE]

this seems to be a pretty typical pony problem. but its usually caused by being ridden by kids who pull on their mouths to much. not saying your daughter is, but anyone who used to ride this pony to make him hate having his mouth touched. maybe have your daughter ride on the buckle and see if he still does it, he may just be saying "please let go of my mouth" also i would maybe check the ponies teeth and bridle fit as something may be bothering him.