Button Press (Not Remote) Normal Door Bell Makes Slight Buzz Only

I have a normal door bell which for some time now refuses to ring from the metal bell on the wall inside the hall. I have replaced batteries and onto my third (outside) door button push bell now but cannot find what the problem is as all it does is just make a slight buzz when the bell is pushed which cannot be heard. It operates by the old fashioned metal bell on the wall inside the hall where the ringer tings inside the outer metal casing which makes the ringing noise. Having constantly changed batteries and bell pushes on the door I thought it may be something to do with the bell ringer itself as I was getting this soft buzz when the button was pushed which meant there was contact there somewhere. I tightened up the screw on the outside of the metal bell in the hall and for once (and once only) it actually rang as a bell should but we are now back to the soft buzz again when the outside door bell is pushed. What could be the problem here? Surely there is not much that can be at fault with the actual metal bell inside the house but it seems that is where the problem must lay having rung once. Does it matter whether the metal casing is hanging losely over the ringer or tightend up tight with the screw for it to make contact with the metal to ring properly or is it a whole different problem?