funny with the bit

[SoulsSerenade@aol.com]

Hi ridecampers.....I should be in school right now....but I don't feel 
well....so, I'm writing to RC instead.
Okay, here's the situation:
I have a 6 year old mare.  I ride her with a regular ol' 5" dee ring snaffle 
bit.  And it is the perfect bit when I ride her....but when I go to put it in 
her mouth, she acts a little strange.  She takes the bit very easily....but 
then she drops her head down and opens her mouth and the bit sort of hangs 
there.  I thought that it was too loose so I tightened it..but then, when she 
held it in her mouth, it was too tight.  So, I put it back on the original 
settings.  When it is on the original settings, and she holds it in her 
mouth, like a good horse should, it appears to be "lookin' good"...but then 
she opens her mouth again and it falls down.....I'm afraid her tongue is 
going to go over the bit.  
And when I ride her, and we are just walking along, relaxing, she does it 
again.  So I kind of give a little tug...and that usually makes her take the 
bit in her mouth again.  But sometimes she just keeps walking with her neck 
streched out and her mouth open and the bit hanging there.  
What could this be?  Could she have a problem with her teeth?  Should I try 
another bit?  
thanks
-Renee 
and Sere (shhh..I'm just tricking her)