I simply do not understand the science of drilling holes in walls and
then getting the right-sized plug to fit. This morning I drilled the
first hole for a towel rail, and fitted the plug - no problem. Then I
drilled the second hole - same drill bit for the same sized plug.
However, when I came to insert the plug, would it go in? No it would
not. Why does this happen? How do you match the drill bit, plug and
screw? The usual answer is that you look for the numbers given on the
plugs/screws/bits. But what if these numbers are not present? How do
you do it by eye, which would be more convenient anyway.

Any answers appreciated.
In limine sapientiae