get_row returns empty when data exists

You have several things going on there and I may not be able to sort them all out.

  1. Unless $wpdb->cf7dbplugin_submits has been added to the $wpdb
    object your query won’t work. You will need something like
    {$wpdb->prefix}cf7dbplugin_submits instead but I can’t really
    guess at the right value.
  2. You don’t need to swap in 9999. That is a hard-code value. Swapping
    it in with prepare is a waste of resources.
  3. Prepare accepts 3 different placeholders, one for a string, one for
    a digit, and one for float. By using %d you are truncating your
    decimal. You need %f.

Perhaps something like this will work better:

$query =
"SELECT submit_time, form_name, field_value" .
"FROM {$wpdb->prefix}cf7dbplugin_submits " .
"WHERE field_order = 9999 AND submit_time = %f ";

$queryp = $wpdb->prepare($query, $postid);

Item #2 won’t cause a failure of the query, but item #1 would, so that is where I am betting this goes wrong. However, item #3 would just as likely break the query as you would not get a match on a decimal value if the decimal input was truncated to an integer.