Help needed to solve incorrect tiling of thumbnail posts rows

Hello,
I posted this under another post by mistake…
I am quite new to wordpress and no coder at all, but I tried to use the code from a previous post by jiinlaw and adapt it to my needs.
I have a list of posts by just one category and it all kinda worked except for 2 problems:
1) thumbnails don’t tile properly in the page
2) the nav shortcode results in a misplacement of the navigation elements

Note that 1) happens irrespective of nav being in the code or not.

my page is: https://genmic.unipv.it/linee-di-ricerca-wp-list/

and the plugin code for the list is

[posts]
<div class=”postItem”>
[post_thumbnail size=’thumbnail’]
<h6>[title]</h6>
</div>
[/posts]
[nav type=”plain” ajax=”1″ prev_text=”Precedente” next_text=”Successivo”]

The CSS Style bit is

#w4pl-list-[listid] .postItem{
float: left;
margin:0 1% 30px;
}
@media screen and ( min-width: 480px ) {
#w4pl-list-[listid] .postItem{width:30%;}
}
@media screen and ( max-width: 480px ) {
#w4pl-list-[listid] .postItem{width:98%;}
}
#w4pl-list-[listid] .postItem img {
float: left;
margin: 0 0px 15px;
}

I tried for quite a long time but cant fix it.

Any help is appreciated ;-P

thanks

https://www.ads-software.com/plugins/w4-post-list/