1. Be sure that her teeth have been floated within the past year. Pain associated with the bit hitting any teeth that need filing down or sharp teeth hurting her tongue or the inside of her cheek can cause a horse to refuse the bit.
2. Be sure you are not causing it by banging her teeth at all when bitting up or taking the bit out. If the bit hits the teeth it's going to cause refusal because it hurts.
3. Be sure that when you ride, you're not pulling on her mouth a lot. Horses soon figure out the cause of their discomfort or pain and they refuse to take it anymore. So, if you're heavy handed, this can also be a reason why she's refusing (if you can check off all the other possibilities). For example, a horse might suddenly be hard to catch if the saddle doesn't really fit and is causing pain/discomfort. The problem isn't that the horse is hard to catch, but that the saddle doesn't fit. Fix the saddle issue and the catching issue goes away.
4. Did you really take the time before to of trained her to take the bit? Lots of times, when horses take the bit but aren't really taught to...they get to a point where they say, no. And since they don't have any real training....they just flat refuse. It's like a horse that's always trailer loaded without officially being taught to...and one day decides not to...the owner is perplexed and frustrated, but the horse refuses simply because the horse doesn't really know (like if a horse is bothered by something else...for that reason, the horse might just refuse).
Training to a horse means they do what you ask even when they don't want to...they do it because they know it's what's expected of them. A cue. They do it because they trust that they will be rewarded with a release of pressure (once the bit is in the mouth, you stop fussing over it. That's the horse's release of pressure for that lesson).
How to fix it: (using loads of approach and retreat)
1) teach your horse to drop her head on command. Once you can ask her with minimal pressure (be able to put your hand on her neck and she drops her head easily and keeps it down for a few seconds)
When you can do that then add:
2) be able to cup her mouth with your hand as if you had a bit. You may need to start by being able to rub her face all over with your hands.
3) be able to stick your thumb into her mouth...over the bars (no teeth area) and she opens her mouth wide enough for a bit every single time.
4) using a lead line. Put a loop in a lead line and pretend that is your bridle and bring it down between her ears and use the loop part of the loop to be your pretend bit. This way, you can practice bitting up the horse as many times as necessary without using a real bit and you don't risk smacking her teeth with the bit if she raises or throws her head.
5) only when you can go through each of these steps without skipping and if she raises her head at any time, go right back to step one and on from there. Don't rush. ..... only when you can do all of em and she takes the pretend bit, then replace that with the real thing.
6) be sure to reward the slightest try, so if that means being able to hold the bridle over her face (between her ears) without her moving her head, then take it away and reward her with leaving her alone for a moment then build on that til you can actually go through all of the steps above and get her bitted up. Always be sure to ask her to open her mouth wide enough to take the bit. (use your index finger to raise her upper lip and your thumb to open her mouth)
Don't use food! I've got a client who never officially taught her horse to take the bit, but instead would put molasses on it and now guess what? The horse is starting to refuse the bit and the owner sometimes has accidentily banged her teeth. So....a real cue must be taught to the horse if you want your horse to accept the bit every time without issue.
If a horse knows what's expected of him or her, they'll do it. If they're being taught to do it in a fair way. That is, no banging the teeth and no force, no hurry up and get the bit in before she raises her head...... But step by step....let her get comfy with the cue....and be consistent in how you handle it from now on and she'll always take the bit.
Also, when you remove the bit, always put your left hand over her nose to point it downward and let her spit out the bit so this way, you don't bang her teeth by taking the bit out and she raises her head....