If they are huddled as far away from the heat as they can get, it's too hot. Feel the plate. If it burns your hand, it's a problem. Just had another thread a couple days ago where the heat plate was too hot and it killed most of the chicks because they wouldn't go under it and got chilled.
Check the "footprint" with a thermometer. Lay it on the floor under the heat plate. If it reads more than 90F, you probably should ditch the thing. A heating pad set-up would be a lot safer and wouldn't get so hot.
The chicks should be freely moving from heat to cool space, back and forth as they need to replenish lost body heat. Huddling in one spot indicates a problem.
Your brooder may be too small to allow for this free movement. Chicks are like reptiles at this age, regulating their body heat by moving in and out of the heat zone. They need plenty of room in which to do this.