Css help, please. WP stylesheet is missing some declarations

There are CSS declarations that I have added to the styles.css file that are not used in the WP page. For example, I have
#page_div .bookThumb {
width: 135px;
min-height: 140px;
vertical-align: top;
font-size: .7em;
line-height: 1em;
text-align: center;
background: none ;
display: block;
}
but the styles page that is used by WP has only

.bookThumb{
padding:10px;
}
which, as you can see, isn’t even one of my codes.
And all of the styles after this one are dropped. Needless to say, my page looks really bad.
I am new to WP, and would appreciate any help
Thanks