Keep all footnotes on the according page

Latex attempts to do what you are looking for, but sometimes it thinks that the result would be better otherwise. For example: if you put a footnote on the last line of a certain page (or rather what was the last line before you added the footnote), where should the actual note be? there is no room to place it in the original page, so it must be in the next, but then the last line must also move to the next page, which creates badness (due to empty lines in the page).

Read here: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=splitfoot for a way to change the penalty of a "split foot" (ouch!) and thereby, perhaps, alleviating your problem.

The idea is to give the command \interfootnotelinepenalty=10000 at the beginning of the document (the default is 100). Note that you might want to give the command \raggedbottom (double ouch!) so that you will no get error due to the empty lines you will no doubt get.

20 to 30 footnotes per page is unusual, so it's not surprising that default latex classes don't handle it well. The footmisc package has a feature (option para) to avoid starting a new line for each short footnote, which may help a little.

\dimen\footins (default 8truein) gives the maximum amount of space that Latex will allow for footnotes on one page. You can try increasing this.

You should also set \raggedbottom and \addtolength{\topskip}{0pt plus 10pt} to help tex look for page breaks before it's too late. (See this FAQ answer for the reason behind the stretchy \topskip).

Critical editions typically have a large number of footnotes, so possibly ledmac or one of the other packages for that purpose will have some features that can help you (I haven't checked).

TeX does not usually detach footnotes. If they happen to be split off, this is usually due to some formatting problem like a manually specified pagebreak detaching the footnote.

There is one particular case where TeX will move a footnote to the next page: it tries splitting some footnote to get room on the current page. Whether it succeeds or not, the next footnote will float to the next page.

So the "missing footnotes" situation occurs mostly in situations that can't be salvaged using TeX's algorithms anyway, when you have multiple footnotes anchored on one line, and not even the next before last one will fit.

A manually placed linebreak before the floated footnote anchor might be inconspicuous if the paragraph before it is long enough.

The combination of in-paragraph footnotes and the less encumbered page breaking of the bigfoot package will usually defuse a lot of situations for critical editions. But if you have a lot of footnote anchors, picking a few manual linebreaks (or juggling with \looseness) might still be necessary on a handful of pages per thousand: TeX does not reconsider its linebreaks in the main text based on footnote layouts, and even bigfoot does not meddle in that area.