Open your postbit or postbit_legacy template. At the left you'll see a link called "Show Default"; click it. Look for this code:
Code:
<if condition="$show['inlinemod']">
<input type="checkbox" name="plist[$postid]" id="plist_$postid" style="vertical-align:middle; padding:0px; margin:0px 0px 0px 5px" value="$post[checkbox_value]" onclick="inlineMod.toggle(this)" />
</if>
Check that code against your current postbit template. If it's missing then add it back.