Ever the optimist I have another go at soya, both indoors and out, every couple of years. And like you I just get a handful of pods. As far as I can tell it is more a day length than a temperature problem. Edamame is a Japanese crop and although a great deal of soya is grown in Hokkaido, where weather is not that different from NE England, it is still a good bit further south so day lengths are different. There are now varieties that yield quite well right across southern England, ten degrees north of Hokkaido, but they are all being bred for commercial field scale bean production and are only available to large scale commercial growers under very strict licensing conditions. (We are in “grow this without a license and we sue” territory here. Like GM crops.) They also like pretty light soil - are you growing them in raised beds or on the flat in the polytunnel? They do a little better for me on raised beds. My raised beds, indoors and out, are currently standing in six inches of water. Meant to pick the last of the tomatoes today but there’s a hole in my wellies below water level.