Veggie Problems

Ok so i admit i am a newbie. So far most everything i have cooked on my LBGE has turned out good.. However i have tried grilling squash twice and it turns out bitter. The first time i did not let my lump burn long enough. Tonight a had a lot of lump left from a previous cook and i added a small amount of lump to it. I fired up the grill and let it burn for 45 min. I left the lid up for about 20 minutes then i closed the lid adjusted the bottom vent to about 1/2 inch. Took the daisy off. Stabilized the temp to 400 degrees. I cooked the squash and grilled chicken breasts. The squash was bitter and the chicken was good. Please let me know what i am doing wrong.