It definitely looks like a WordPress theme issue to me (although I could be wrong)

It also looks like you have more forum hidden under the WordPress page header than just the dialog box. All I can see is from “Advanced Search” down, and there should be a lot more above that before you hit the start of the header.

Can you provide a link to your site if it’s live?

Never mind I found the site through Google, it’s the way the screenshot was taken. Ok…

So there is a bar at the top of your WordPress theme that sits on top of the forum, which has a: z-index: 9998

You will need to open your simple-press folder, navigate to forum-themes which is typically simple-presswp-contentsp-resourcesforum-themes and find the theme you are using.

If its Default for example you want to open forum-themesdefaultstylesdefault.php

run a search for ui-dialog, and you should have:

.ui-dialog {
    position: fixed !important;
    padding: .2em;
    width: 300px;
    background: <?php echo($alt4BackGround); ?>;
    border: <?php echo($alt4Border); ?>;
    color: <?php echo($alt4Color); ?>;
    <?php echo($largeRadius); ?>
    font-family: <?php echo($dialogFontFamily); ?>;
    z-index: 10;
    margin-top: 50px;
}

Can you just try changing the z-index: 10;    to  z-index: 9999;

This should enable the ui-dialog windows to display above the top bar in your WordPress theme

Ike, it worked!  You are a scholar and a gentlemen.  Thank you kindly.

No problem glad it helped

You might want to look at turning your theme into a child or custom theme, or when there is an update you could loose your fix as it could be overwritten.

Take a look at Child Themes and Custom Themes

Either method will preserve your change, or you could simply note the fix, and if it ever gets overwritten with an update you know how to rectify