I've only had my brooder plate for three batches of chicks so far this spring, and have learned they will reject it if it's too low. It has to be high enough so that they must stand up fully to press their backs to the plate, and even higher than their backs as they grow a bit older. I keep one end a bit lower than the other, too.
Also, it's a possibility that the surface temperature of the plate is off. I *think* from what I've read, the surface temperature should be about 125-deg-F. Just hot enough to feel uncomfortable if you press your hand to it, but not hot enough to burn you. (Chicks have their down to insulate them from the heat.)
My plate was HOT enough to burn my hand; no wonder they rejected it! But with chicks in the brooder and another batch ready to hatch soon, I had no time to order another or complain to the online store. So instead I ordered a temperature regulator, and hooked that up to the plate. Now it holds at 125-deg-F just fine, and when adjusted to the right height, the chicks use it most of the time at night, occasionally during the day. They'll still bunch up around the outer edges of the plate, but I've got it placed in the brooder where heat from the plate may be reflecting off the brooder walls around it (about 4-6 inches apart). So it makes a snug little zone for them to huddle, and not be under the plate.
As long as they're quiet and active, I don't worry about it. They'll peep LOUDLY when they're too cold or too hot.
