I have used a pre-defined position for the Automatic Related Products 2 block, but the block does not show up.
Such issues can appear because of the peculiarities of a custom theme that may be in use at the store.
The Automatic Related Products 2 extension uses native Magento blocks to attach its own blocks to. The module recognizes native blocks by names.
In some custom themes, certain native blocks may be either missed, or have non-standard names. In such cases, the ARP 2 extension is not capable of finding a necessary block to attach its own one to.
For example, the Automatic Related Products 2 extension features the "Instead native related blocks" and "Under native related block" pre-defined positions. If the native related products block is missed in your theme or its name differs from the standard one, the ARP 2 extension is not able to find the target block and add its own block to the required position. In such a way, the pre-defined block positions above don't work.
There are several possible solutions:
1. use the 'Custom' position of the Automatic Related Products 2 module - in this case, you insert the ARP 2 block manually, it becomes independent from any standard blocks (you can find the details on how to use custom block positions in the extension readme).
2. customize the extension to make it work with your current theme - it mostly depends on the code of your theme, but you will have to do this again for every extension upgrade.
3. customize your theme - this implies adding missed blocks into layout and templates and setting their names to default values.
The latest is the best option, because it also provides a compatibility with other modules you may want to install afterwards. All Magento developer companies create their modules on default theme and rely on some native Magento features, which may be missed or work incorrectly in custom themes.
In case you would like to use our technical services for fixing such problem, please note the following: implementing any of the solutions above may come at a separate price, as they require some extra work which goes beyond standard support conditions. The actual price depends on the code of your theme.
The same applies to Automatic Related Prodcts for Magento 2. However, new extension provides more flexible usage options for admin: All kinds of different blocks and their positions are displayed on one page, which makes a management of the blocks easier and more straight-forward.