Poor subsoil conditions. Have you done a compaction/perk test? Take a shovel or a post hole digger and walk your site. Dig or auger a series of holes a minimum of 2 feet deep across the site. Check for orange iron banding in the soil which could indicate a seasonally high water table. Do the holes fill up with water? If they do, you may have to install drain tile or sockpipe. Also, note any "hardpan" - soil layers that are so compacted that roots can't penetrate. (If you have difficulty digging through the soil, imagine a plant trying to put a root down into it.) Hardpan is really common in agricultural land that has been field-cropped with tractors for many years.
Prior herbicide applications to your site. Do you know what chemicals were applied to your site? Has Atrazine, dicamba, 2,4-D or any generic version of these ever been applied? These herbicides are very persistent and slow to break down. Atrazine, in particular, has a half-life of over 25 years. It simply leaches and moves deeper into the soil strata. It takes 3 to 5 years just to leach out of the top 6 inches of topsoil. (Now it is starting to show up in well water.) If you have a site with past applications of these herbicides, you can expect damage to the Hop plants ranging from slight yield reductions to outright death. The hop plants may start out fine, but when their root system reaches the contaminated zone expect trouble. Unfortunately, there is not much you can do, other than select a new clean site. You can have your subsoil sampled and tested prior to planting, to be certain. A possible visual clue is to check out any trees growing around the perimeter of the site. If they appear sickly with dead branches and distorted or stunted growth be aware their deeper roots are probably picking up the residual herbicides.
In short, it pays to investigate these things before you start . . . Thanks for reading and hope this helps your hop growing.