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Given that your beech is struggling to grow, I would actually recommend a hard pruning/renovation, which beeches respond well to. It can all be reduced by as much as 50 percent in height and width in a single cut. It will take a few years to be restored to its former glory but you should have a healthier more vigorous hedge. For more information, please visit the following link:

https://www.gardeningknowhow.com/ornamental/shrubs/shgen/rejuvenation-pruning-tips.htm
https://extension.psu.edu/plants/green-industry/news/2012/hacking-whacking-dealing-with-overgrown-shrubs

This plant should be spaced at 2 feet apart to 2.5 feet apart. The closer spacing will form a dense hedge more quickly, while the wider spacing will eventually form a dense hedge and will allow you to cover more ground with your plants.

This appears to be a fungal or bacterial issue. There are a few things that you can do. I have a recipe that I use that is very broad, powerful, and most of all... safe. If you don't have every ingredient, it is ok, but please don't skimp on soil priming. It is important.

The recipe is as follows: First, you will want to prime the ground with dolomitic lime, then spread wettable sulfur to Kill off any infection in the soil. I Also like to spray the plant itself. Mix a few drops of one or more of these ingredients into warm water with a spoonful of coconut oil: Rosemary oil, lavender oil, neem oil, garlic oil, cinnamon extract/oil and for really bad cases, also add clove oil.

For other helpful information please visit this link: https://www.gardeningknowhow.com/plant-problems/disease/root-rot-in-garden-plants.htm

I appears overwatered. If it remains wet in your country or area, then this will be the culprit. The best thing to do from here will be to put something in for preventative measures.

My recommendation is a 1/4 cup of DOLOMITIC LIME and a Teaspoon of WETTABLE SULFUR per shrub. Scatter this around the soil to prevent future infection.

For now, you should treat with a fungicide. This article will help: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

The growth will grow back once it is established, and healthy. This article will help you to care for Beech Trees regardless of whether it is tree or hedge form: https://www.gardeningknowhow.com/ornamental/trees/beech/beech-trees-in-landscapes.htm

You might try skipping the August trim and see how fast it grows. Then in February, give it a trim to tidy it up and encourage more growth. Here is more:
https://www.gardeningknowhow.com/ornamental/trees/beech/prune-beech-hedge-trees.htm