Hi Marsha!
I haven't had this issue last long w/ any of our driving ponies. HOWEVER, I have with a riding pony. Didn't matter what bit we used (there are many more choices now then there were 10-15 yrs ago that we could afford or that fit the pony), she insisted on putting her tongue over the mouthpiece and then sticking her tongue out - often waving it around... tried dropped noseband, flash noseband, figure 8 noseband & regular cavesson - all tried very tight and correctly adjusted. Didn't matter - she could get her tongue over the bit and seemed to delight in our puzzlement/frustration!
When riding at home, we went to using a nylon/chain curb strap (yes, the western one) and put it on one bit ring, over her nose and attached to the other bit ring. Had to add holes to make it more adjustable. One w/o a chain would have worked too, I think, but we didn't have one at the time. This raised the bit off of her tongue AND she could not get her tongue over it. However, this changes how the bit works - it no longer works as effectively. Also did the "cowboy method" of tying a string to the center of the bit - one side run out of her mouth on each side, brought up over her muzzle and tied in a knot then both strands run straight up her forehead and criss-crossed over the browband and then tied in a bow at the crown. It was similar to the curb strap over the muzzle. Both worked well enough - at least keeping her from putting her tongue over the bit.
We tried the tongue depressor attachment you can get from many english riding stores, but it was too large - she wouldn't/couldn't close her mouth when it was attached to the bit. Acted like she was gagging but even when she shut her mouth it wouldn't close all the way w/ that sitting "in the way".
When she was being shown - we constantly put her tongue back where it belonged. Before a class, we'd check her tongue and then tighten the cavesson. It helped, but didn't prevent her from putting her tongue over the bit - sometimes we undid it again as soon as she came out and then sent her back into the ring for another round (jumping) or for next class (jr classes often ran consecutively). When sold, her new trainer consulted us when she couldn't deal with it - and ended up doing what we did...
As she got older, it DID lesson, while she was being shown (think she got tired of the "game") but if she went more than a week w/o being ridden/lounged in her bridle - the habit came back healthy and hard. The habit never went completely away and even now in her late teens (the last time I spoke w/ trainer), she still does it...
Sorry to be discouraging...
Out of all the horses & ponies we've worked with (maybe not as many as some on this forum), she is the only one that didn't grow out of the habit. However, all of her full sisters (4 of them) started w/ the same habit and it just eased away as they got used to the bit/bridle and work (1 other became a riding pony and 2 became driving ponies - all at large event competition levels. After starting w/ habit, I lost track of the last one). Several of her 1/2 siblings by the same sire also started out with the "problem" and again they "grew out of it"...
Here is Vixen's 1st ride (she'd been ground driven and had several hitching to cart and driving previously). Not sure her tongue is actually over the bit here, but it definitely was previously and we did have to fix it during her short ride. She also sometimes bruised heavily when she put her tongue over the bit. She learned to give to the bit and respond to reins - even when her tongue was over the bit. Sometimes I swear she was more comfortable that way...
Her tongue is over the bit here - and we fixed it...(had already done so several times this day):
This one shows the curb strap over the nose. Sierra is riding, ponying Vixen's 1/2 sister by an arab stallion who did not ever develop the same habit as the 5 1/2 sisters sired by our Shetland stallion.
She's playing with the bit here in the lineup, but I LOVE this pic!
The habit didn't stop her jumping ability!
Sierra and Vixen were dubbed the "flying fleas" at this show series for a long time! Vixen was 46" tall and this jump is 2'9" (33")...