In case it can help this is a sample of the xml code.
<xml version="1.0" encoding="UTF-8">
<rss version="2.0"
xmlns:excerpt="http://wordpress.org/export/1.2/excerpt/"
xmlns:content="http://purl.org/rss/1.0/modules/content/"
xmlns:wfw="http://wellformedweb.org/CommentAPI/"
xmlns:dc="http://purl.org/dc/elements/1.1/"
xmlns:wp="http://wordpress.org/export/1.2/"
>
<channel>
<wp:wxr_version>1.2</wp:wxr_version>
<item>
<title>Velvet jacket M</title>
<link></link>
<pubDate>Mon, 15 Apr 2013 17:34:17 +0000</pubDate>
<dc:creator><![CDATA[acreatorname]]></dc:creator>
<guid isPermaLink="false"></guid>
<description></description>
<content:encoded><![CDATA[This is my text. It also includes commas and letters with accets.
<img alt="Velvet jacket M" src="http://www.sample.come/files/ximage.jpg" width="300" />
]]></content:encoded>
<excerpt:encoded><![CDATA[Velvet jacket, no sleeves, silver buttons...]]></excerpt:encoded>
<wp:post_id>1</wp:post_id>
<wp:post_date>2013-04-15 17:34:17</wp:post_date>
<wp:post_date_gmt>2013-04-15 17:34:17</wp:post_date_gmt>
<wp:comment_status>closed</wp:comment_status>
<wp:ping_status>closed</wp:ping_status>
<wp:post_name>velvet-jacket-m</wp:post_name>
<wp:status>publish</wp:status>
<wp:post_parent>0</wp:post_parent>
<wp:menu_order>0</wp:menu_order>
<wp:post_type>post</wp:post_type>
<wp:post_password></wp:post_password>
<wp:is_sticky>0</wp:is_sticky>
<category domain="category" nicename="jackets-and-coats"><![CDATA[Jackets and Coats]]></category>
<wp:postmeta>
<wp:meta_key>Model</wp:meta_key>
<wp:meta_value><![CDATA[X10]]></wp:meta_value>
</wp:postmeta>
<wp:postmeta>
<wp:meta_key>Quantity</wp:meta_key>
<wp:meta_value><![CDATA[1]]></wp:meta_value>
</wp:postmeta>
<wp:postmeta>
<wp:meta_key>Price</wp:meta_key>
<wp:meta_value><![CDATA[60.0000]]></wp:meta_value>
</wp:postmeta>
</item>
</channel>
</rss>
I have found another post that gave me some clue about the possible solution.
The issue was likely related with post having the same date and mostly the same pos_id tag.
So it is important that all posts in the xml file have different times (pubDate, wp:post_date, wp:post_date_gmt) and post_id or, as I did, no post_id tag all.
🙂