Wordpress posts disappear in index page

Question

I have been creating a one page theme for a wordpress site from scratch, and in my local server, it displays all posts related to the sections normally. However, I find sometimes that one post, usually the starting one out each category of posts I have maintained, will not display data. what can be the reason for this effect? Please comment if you need to see an excerpt of the code I have maintained.

Edit:- Here is an excerpt of what I have done in index.php:-

<div id="img_slider" class="row">

                <?php if ( have_posts() ) : while ( have_posts() ) : the_post(); 
                    if (in_category(3)){ ?>


                        <?php the_content(); ?> 


                    <?php } ?>

                <?php endwhile; else : ?>

                <?php endif; ?> 

            </div>

            <div id="message">

                    <?php if ( have_posts() ) : while ( have_posts() ) : the_post(); 
                        if (in_category(5)){ ?>
                            <div class="box">

                            <?php the_content(); ?> 

                            </div>
                        <?php } ?>

                    <?php endwhile; else : ?>

                    <?php endif; ?> 

            </div>
            <div id="products">
                <div class="grey-box" >
                    <h2>Product List 1</h2>
                    <div id="products-main">
                        <div class="row">
                            <?php if ( have_posts() ) : ?>

                            <?php while ( have_posts() ) : the_post(); 
                                //echo '0';
                                if (in_category(2)){
                                    get_template_part( 'template_parts/product');   
                                }   

                                ?>
                            <?php endwhile; ?>  

                            <?php else : ?>

                            <?php endif; ?>
                        </div>
                    </div>


                    <h2>Product List 2</h2>
                    <div id="products-other">
                        <div class="row">
                            <?php if ( have_posts() ) : ?>

                            <?php while ( have_posts() ) : the_post(); 
                                //echo '0';
                                if (in_category(6)){
                                    get_template_part( 'template_parts/product');   
                                }   

                                ?>
                            <?php endwhile; ?>  

                            <?php else : ?>

                            <?php endif; ?>
                        </div>
                    </div>
                </div>
            </div>

Answer 1

I found the answer to my question. The problem was with the number of posts that the loop could handle at a time. If you need to increase or decrease the number of posts the loop can handle per page, add this code BEFORE the loop.

<?php query_posts('posts_per_page=amount'); ?>

<?php //the-loop-goes-here ?>