LaTeX knows where it's best to put figures. It is right. Sometimes things are not what you want them to be.
use [htbp] as the qualifier for any floating environment (e.g. figure or table). This will make LaTeX try it's best to put a figure close to where you want it.
LaTeX algorithm puts out figures in order and when it can find an appropriate space. If you have a lot of figures needing a lot of space compared to the space needed for text, then they could be saved up and printed at the end of a chapter (or document).
\clearpage will instruct latex to put out the floats now and start a new page. This may leave a blank and of page, but this is sometimes better than the alternatives.