The standard LaTeX \footnote command doesn't work in tables; the tabular environment (and its “relations”) traps footnotes, and they can't escape to the bottom.
If one has both tabular-only environment and tabular inside table, it is possibile to . You can also use threeparttable package to insert footnotes in the tables.
This is a classic difficulty in LaTeX. The problem is how to do layout with floats ( figures and tables, an similar objects) and footnotes.