You specified placement '[htbp]' for your full-width figure or
table, but they always get placed at the top of the page... Well,
it is what the documentation says: LaTeX, unadorned, only
allows full-width floats at the top of a page, or occupying (part of) a
float page.
The stfloats package ameliorates the situation somewhat, and makes LaTeX honour '[b]' placement as well; the dblfloatfix package combines a tidied version of the changes made in stfloats with the float ordering corrections defined in fixltx2e.
A particular problem with stfloats and dblfloatfix is that the float will appear, at its earliest, on the page after it is specified. This has two undesirable side-effects: first, there may be no bottom float on the first page of a document, and second, float numbers may become "entangled" (particularly if you're using dblfloatfix that ensures that the early-specified bottom float is set before any single column floats).
(The FAQ team doesn't know of any package that will make LaTeX honour '[h]' placement of double-column floats, but the midfloat package can be pressed into service to provide something approximating the effect it would have.)
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=2colfloat