width input field stuck

Hi,
I’m using this plugin on some sites I built. It works very fine, except on the last site I’m building now: for some reason the width of the input fields cannot be changed, the width is the same (narrow, for about 20 digits) for every type of input field.
I tried everything, like changing css in customizer, changing css in child theme, adapting fields in contact form itself, and of course changing through CF7 customizer, but even changing to full width for the input fields in CF7 does not change a thing (on any browser).

Disabling all other plugins does not help either. I think this is caused by the theme (although I use this theme on the other sites where this problem does not occur).
I posted a support question (https://www.ads-software.com/support/topic/input-field-width-stuck/), but without reaction so far.

I wonder if someone on this CF7-forum might have a solution for this.

My site is in the process of being built, so I cannot provide a link. Firebug shows the following code:

<div role="form" class="wpcf7" id="cfc" lang="nl-NL" dir="ltr">
<div class="screen-reader-response"/>
<form action="/contact/#wpcf7-f67-p46-o1" method="post" class="wpcf7-form" novalidate="novalidate">
<div style="display: none;">
<input type="hidden" name="_wpcf7" value="67"/>
<input type="hidden" name="_wpcf7_version" value="4.6.1"/>
<input type="hidden" name="_wpcf7_locale" value="nl_NL"/>
<input type="hidden" name="_wpcf7_unit_tag" value="wpcf7-f67-p46-o1"/>
<input type="hidden" name="_wpnonce" value="857f15279a"/>
</div>
<p>
<label>
Uw naam*
<br/>
<span class="wpcf7-form-control-wrap your-name">
<input type="text" name="your-name" value="" size="40" class="wpcf7-form-control wpcf7-text wpcf7-validates-as-required" aria-required="true" aria-invalid="false"/>
</span>
</label>
</p>
<p>
<label>
Uw e-mailadres*
<br/>
<span class="wpcf7-form-control-wrap your-email">
<input type="email" name="your-email" value="" size="40" class="wpcf7-form-control wpcf7-text wpcf7-email wpcf7-validates-as-required wpcf7-validates-as-email" aria-required="true" aria-invalid="false"/>
</span>
</label>
</p>
<p>
<label>
Onderwerp*
<br/>
<span class="wpcf7-form-control-wrap your-subject">
<input type="text" name="your-subject" value="" size="40" class="wpcf7-form-control wpcf7-text wpcf7-validates-as-required" aria-required="true" aria-invalid="false"/>
</span>
</label>
</p>
<p>
<label>
Uw vraag of bericht
<br/>
<span class="wpcf7-form-control-wrap your-message">
<textarea name="your-message" cols="40" rows="10" class="wpcf7-form-control wpcf7-textarea wpcf7-validates-as-required" aria-required="true" aria-invalid="false"/>
</span>
</label>
</p>
<p>
<input type="submit" value="Verzenden" class="wpcf7-form-control wpcf7-submit"/>
<span class="ajax-loader"/>
</p>
<div class="wpcf7-response-output wpcf7-display-none"/>
</form>
</div>