Force the second popup to display when there are back-to-back popups.

popup-maker-force-back-to-back-display.css

/**
 * If the popup's Display > Advanced > Stackable setting isn't working
 * for some reason, use this CSS to allow the second popup to display.
 */
.pum-overlay.pum-active,
.pum-overlay.pum-active .popmake.active {
    display: block !important;
    /* opacity: 1 !important; If needed. */
}

Also fixes the problem when a popup doesn't display but the page is "frozen" or "blocking" as if the popup is there.

Again, try to set the popup's Display > Advanced > Stackable setting first.

https://wordpress.org/support/topic/popup-not-showing-and-blocking-front-page/