My Arab/Saddlebred gelding has always chewed on bits. I started him in a snaffle. I tried a few curb and shanked bits, but eventually I just put him in a Little S hackamore.  And 12 years later he is still using that bit and doing well with it. He just never liked haveing something in his mouth.

I do use a Imus Comfort bit on one of my horses and I use a good quality correctional bit on some of the others.Some horses respond to tinkering around with different bits, Some horse just don't seem to care and give you the same response regardless of the bit.  I guess that why there are so many bits to choose from.

Do you remember if the original owner rode with a caveson? If they did, they may have had it so tight that she couldn't open her mouth to "fiddle" with the bit or stick her tongue out. She is still a green horse in regards to training not to mention a 6 year-old is still considered a young horse to me. And she could also be nervous from not having too much done with her and exhibiting the anxiety through the mouth.

Like everyone else who posts it is just a guess at the issue since we can't see her. But I'll offer some ideas to think about.

I'd check to make sure she doesn't have any mouth issues like a missed hook on her teeth or a sore neck/back. Or she may remember the pain from having bad teeth prior to the float and has developed this habit. Don't you wish she could talk to you. :)

Once all health issues check out, I go back to a snaffle for training purposes. I like Myler like other posters have mentioned because you will avoid the nut cracker effect in a regular snaffle. Also since she has not been ridden much, she mostly likely needs more basic training to give to the bit along with bending/flexing etc. She needs to develop mucsles and become more fit. Overtime, she will accept the bit as long as you maintain soft hands. The more saddle time you can give her, the better as with all horses.

I have found that most gaited training skips a lot of the basic skills and goes right into the "gait" training with a curb bit. You will get a better gait and more control with your horse if you take a step back and start with flexing, bending and the poll, backing calmly etc and then move back into the curb for gait enhancement. If you horse has a great gait right now, this is just icing on the cake.

My 7-year-old MFT just quit chewing the bit this year; she is a high enery horse and needs something to play with so a roller bit worked for me. It will take time, but most horses grow out of it. I would not recommend a tight caveson at this point in her training unless she is sticking her tongue over the bit.

Good luck, I know this is a highly iritating habit.

my3 horses>> Any level one comfort snaffle will work. Here is a link to the different styles: http://mylerbitsusa.com/comfort_bits.shtml

And then I have had success with my fidgety mare using a level one, short shanked bit barrel bit. She likes to spin the roller which is better than her chomping on the bit. She does like a copper mouth piece since it keeps her mouth moist. http://mylerbitsusa.com/barrel_bits.shtml

There is also a good section on this site that goes over selecting the right bit.

I've bought all my bits on eBay and Craigslist to save on money.

I also ride a finished gelding in a short shank Robart bit as Rose suggested. It has nice balance and works great for a finished horse.

Are you sure you have it high enough in her mouth. Seems most people have there bits low.

I'm not one for bandwagons, but I'm a believer in the Imus bit. I'm sure robarts would be similar. My 4 yr old Rocky Mountain/TWH cross wouldn't gait for her life..just paced in a snaffle, 2 different walking horse bits, wonder bit, and a standard short shank curb. I put her in the Imus Comfort bit with shanks and within 3 days, it was like riding on a cloud. She lightened up on her front, tucks her nose, and gaits beautifully.

BTW, I like that bit style so much that I ordered the one from Robarts that looks to be of the same design.           

I had a gift certificate for Robarts so I decided to buy one from them for my other bridle.  Then I wouldn't need to swap out bits for different riding events.

The IMUS  has a roller in the middle of the bit.  maybe your horse is just PLAYING with the roller. It may not be a discomfort or sign of her disliking the bit. But rather just playing with it.

Sometimes it's like a kid with a loose tooth. They just can't leave it alone.