Wheat varieties vary in height, so a different variety might be what you need.

Some of the popular varieties top out at 18-24 inches when ripe, so try a different one.

Aside from the possibility of cultivars (varieties) of wheat grass what about the soil? What is the soil used in your grow box? What is that soil pH and nutrient level? Same questions about the soil in the box rom the farmers market.

They may be using a coir mat and hydroponic style fertilizer.

What is a coir mat? I did a google search for it, and it came up with fuzzy brown doormats. :/

As for hydroponic style fertilizer, would that be the same as adding a little seaweed fertilizer to the medium?

Part of my frustration is that I've watched numerous YouTube videos on the subject, and most of them achieve results similar to the farmer's market stuff without going to any obvious great lengths beyond what I did.

A coir mat, used in hydroponics, is very similar to a doormat, a medium that plants can root in.

Hydroponic nutrient solutions are much more than a bit of seaweed fertilizer. While some YouTube videos are good many forget important information. You may want to look in your local library for a good book on hydroponic gardening.

I've experimented with growing the wheat berries on a layer of both potting soil and home-made compost, and the results have been much better than what I was able to grow using the soilless method. Not quite as easy, but definitely better results.

see this pub, page 14

http://www.academia.edu/7954283/GREEN_HARVEST_SPROUTING_WHEATGRASS_AND_MICROGREEN_GUIDE