I have so far used Lentils for spawn twice: once with B+, once with Oyster Mushrooms.
When I did it the first time (for the Oysters) I just used them straight up, typical soak overnight, drain for 30 minutes, Pressure cook. the mass of lentils was unbreakable. But the Oyster Mycelium loved it. I had to manhandle the mass (through the grow bag) when I transferred the jar into softwood substrate. Not ideal for spawn because it was awkward to break up but the price was right.
The Second time, for the B+ I had about 10% verm in the jar with them. other than that they got the same treatment. The verm helped the lentils be less bricklike, still far from ideal. Growth rate for the B+ was MUCH slower than the oyster, so much so that I probably wont use it for the B+ again. I'm going to increase the verm level by a another 10% for my next go. It was mentioned in my Blue Oyster Mushroom thread to try adding the verm, and again in this thread. if 10% verm was a minor improvement, I'm hoping there is a sweet spot to find. Oyster Mycelium will eat almost anything from what I've read. Also, my lentils still have the hull on, not sure if that made a difference but I figured it would be worth noting.
What type of mushroom mycelium did you try to grow? maybe it was the strain you were working with deciding it didn't like soybean.