I've never had any difficulty filling mine or keeping the butter from dropping out of the bell into the water.
Over the years, I've learned to take a stick of butter out of the fridge/freezer when it looks like the bell will need to be "loaded." I just let the butter sit out, in its wrapper, on the kitchen counter.
By the time the bell is empty, the butter is soft enough to put in. But...and maybe this is what keeps mine IN the bell, I peel the wrapper off the butter, place the butter into the bell so the butter is standing on end, then I place the wrapper over the other end and shove the butter with the palm of my hand on the outside of the wrapper into the bell. This way I don't get any butter on my hands/fingers and the butter is really pushed into the bell. Did this make sense? I can see it in my mind and I hope I conveyed it with some semblance of reason.
"As a girl I had zero interest in the stove." - Julia Child
This is real inspiration. Look what Julia became!