They might be too wet. After you wash your vegetables, pat them dry or allow them to air dry before they go into the oven. More moisture results in more steam, which prevents things from getting toasty and crisp. Or, you might need another sheet pan to space things out. Crowded vegetables also generate steam. Aim for a bit of breathing room between each sprout (employ another sheet pan if needed), and start the Brussels sprouts with the flat sides down to score more of those brown, crunchy bits. Flip the vegetables halfway through the cooking time so the other side has a chance to roast evenly as well.