Most themes default to showing custom blocks at the top, Your theme.html might be set to show blocks below the modules, the code would need to be altered, which theme are you using and I might be able to help further?
See this thread here, although this one was for someone who wanted custom blocks below the modules, the principle is the same though:
https://xoops.org/modules/newbb/viewtopic.php?topic_id=22516&forum=7#forumpost107788
This code in theme.html is the code that determines where the custom blocks apppear:
<div id="content">
<{$xoops_contents}>
</div>