Vegetables release moisture in the oven. When they are piled several layers deep, the moisture lingers around them and they soften without much browning.
Spread the pieces in a reasonably loose layer. They do not need enormous personal space, but there should be enough exposed tray for moisture to escape. Use a second tray if necessary, and rotate positions if your oven cooks unevenly.
Oil should lightly coat the surfaces rather than collect in a deep puddle. Turn when the underside has some color, not every few minutes out of habit. Check tenderness separately from browning: a dark corner does not prove that a thick potato is cooked in the center.
If the vegetables are soft and pale, uncovering and spreading them out may help. If they are already burnt at the edges, raising the temperature is unlikely to solve the remaining hard middle.
When two vegetables need different treatment
A pepper may soften while a thick potato is still hard. You can cut the potato smaller, start it earlier, or roast the vegetables separately. The easiest option depends on the oven space and how much attention you want to give the meal.
Use a tray with a rim to contain oil and juices, and check that any liner is suitable for the temperature. A dark tray may brown the underside more quickly than a pale one. If the bottom is getting too dark, change the position or heat according to your oven rather than repeatedly turning up the temperature to hurry the center.
Once the vegetables are done, move them into a serving dish only when you are ready. Piling hot roasted pieces into a deep covered bowl traps steam and softens the edges. That may be acceptable for a warm salad, but it is worth noticing if crispness was the reason you roasted them.