I have dental implants but have had problems with air blowing through small gap at top of bridge where gum hasn’t healed. How can this be filled?

Q. Hi, Eight of my top teeth at the front are secured by four dental implants. I had these fitted four years ago and have had problems ever since with air blowing through small gaps at the top of the bridge where my gum hasn\’t grown back. My dentist has tried to fill the gaps a couple of times but this keeps falling out. Do you have any suggestions of how to fill the gap? I did think that maybe botox injections in the gum might help but I don\’t know if this is possible. Any help would be greatly appreciated. Many thanks.,