Actually it is not the case the module ignores the parameter. The problem is down to the fact that Joomla splits the article text into two different database fields, introtext and fulltext.
It is not clear from what you are saying which module you are referring to, but to solve it you will need to find the appropriate helper file in the module, and in the code replace
with
$row->introtext.' '.$row->fulltext
wherever you see it in the code.
If you would like to be a bit more specific about which module you are using, and which version of Joomla then I can be a bit more specific about the precise code you need to change.