Figures move around because there's not enough space on a page to fit
them, so it hopes to form a page of figures.  However, it will not
form such a page until it has a sufficient height of figures to set on
a page.  Thus, often figures float to the ends of chapters or
documents.

The solution is to change the float parameters to, for example (from
~rph/tex/archive/float.sty): 

\def\topfraction{0.90}	      % Amount of page that can be floats
\def\textfraction{0.10}	      % Amount of page required to be text
\def\floatpagefraction{0.70}  % Amount a float page that must be filled
\def\dbltopfraction{0.90}
\def\dblfloatpagefraction{0.85}


One other often odd thing happens because tables and figures are on
different float lists:  tables and figures can pass each other by, not
necessarily being set in the same ordering.  If you don't want this,
use ~rph/tex/ftsame.sty, which places figures and tables on the same
float list, but maintains the independent numbering.