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
The package nidanfloat supports
[b] placement of a
full-width figure in a two-column document even on the first page of a
Barbara Beeton's article Placing a full-width insert at the bottom of two columns in TUGboat 35(3) gives a manual method for doing the same.
The stfloats package also ameliorates the situation somewhat, and makes LaTeX honour “[b]” placement as well.
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.)
Prior to the LaTeX2e 2015 release, double and single column floats were maintained in separate lists and could be output in the wrong order. For these older releases dblfloatfix package combines a tidied version of the changes made in stfloats with the float ordering corrections defined in fixltx2e.