1. OWL Slider / Banner not showing in store front. Most common issues.
Most common issue encountered is not using a valid visibility interval.
For each banner you can set a valid visibility interval , valid from - valid to.
This way you can schedule banner visibility with different messages and promotions. However if you set a visibility interval is in the past, your Banner will not show up in your Slider. If you are using only one banner in your slider, and visibility interval invalid, your slider will not show up at all.
If your banner should be always visible you can set a 'Valid To:' date in the future, ex: 1/1/2030.
2. OWL Carousel not Showing up in store front. Most common issues.
In order for New and Sale Product Carousel to be displayed, make sure you inserted the carousel in your page/block and there are some products defined as "New" respectively “Sale” in your catalog. If you have no product defined as as "New" or "Sale" the carousel will not show up.
More details here: How can I set a product as "New" or "Sale" in Magento 2
In order for Best Sell Product Carousel to be displayed make sure you inserted the carousel in your page/block and there are some orders in your store, best selling products are determined based on magento statistics. Make sure Magento statistics are activated and best selling products show up in your magento admin statistics. Also make sure your products are visible in Catalog in order to show up in this carousel - simple products marked as not visible individually, will not show up in this carousel only if product visibility is enabled.
More details here: How to show best selling products in Magento 2 admin or store carousel
In order for Recently Viewed Carousel to be displayed, make sure you inserted the carousel in your page/block. Make sure to use a extension version grater or equal with 1.6.4 (May 16, 2018) when this functionality was fixed.
In order for Related Products Carousels, Up-sell Products Carousels to be displayed on product page make sure this carousel type is Enabled and the product you are checking has at least one related or up-sell product defined in magento catalog.
For Cross-sell Products to be displayed, make sure you have defined Cross-sell products in your catalog for the product you are checking. Add that product to cart and check the shopping cart page.
For Custom Category Carousel to be displayed, make sure you entered the correct category and the category contains at least one product. (option available only in Pro version of OWL Carousel and Slider extension)