Securing the bung

Here's a problem that I haven't seen discussed before.  Sometimes, not
always, when I attach a bung to a carboy it refuses to stay seated.   This
seems to be independent of whether the bung is drilled for an airlock or
not, so I'm fairly sure that it has nothing to do with fermentation.  If the
carboy and bung are very dry then it is less likely to happen.  It also
seems to happen the most with my carboys that have a mouth only a little
smaller than the largest dimension of the bung.

My "method" of securing the bung is with duck tape. Not very convenient, but
effective so far.

Is this typical?  Do you think it will happen less if I get slightly larger
bungs?  Any other thoughts?

Thanks in advance,
Dave