hmmm not so sure that its the bridle's fault - do you have one wrinkle at each corner of his mouth from the bit? If so, the bridle sholud be fine. Sometimes the bit can slide through the horses mouth if you pull on one rein and let the other go slack. Ideally, when you use one rein, the other should still have contact to stop it sliding through.
Hope this helped!
When I bestride him, I soar, I am a hawk: he trots the air; the earth sings when he touches it; the basest horn of his hoof is more musical than the pipe of Hermes. ~William Shakespeare