Wow. I have not had this problem before, so I thought I'd ask for a little help:
My 5 year old gelding and I had a lesson this past Sunday. I noticed right away he was pulling through my hands, and very heavy on the bit. I did not have a problem stopping/backing him up, but just going along at the trot he really wanted to stick his beak out and pull through the bit. Usually when he does that, I bump his nose by alternating reins left right left right. He is excellent to put his head back down and collect after that reminder, but he would only last a few strides before sticking his head up again.
Now, it's been a while since his teeth have been done (the vet is doing them this week) could this be the issue? My trainer also suggested maybe the bit is too soft and suggested the controversial Tom Thumb, just for a few weeks to remind him that behaviour is not ok (I have him in a double jointed copper snaffle currently.) I really feel my hands are not soft enough for the Tom Thumb bit, as I do get a little jumpy sometimes and don't want to yank his head off.
So, any suggestions as to why he is doing this? I am really hoping he was just having an off day, he has been doing really well and was finally looking and moving like a nicely broke horse, and it is not really his bag to be defiant (well, maybe a little naughty sometimes!!) Thanks everyone!