Do you have a feed store near you? You should try and find out if they carry Nutri-Drench, it really does work wonders with weak chicks.
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If the heat mat is under the whole brooder, you have a problem. When providing contact type of heat, which the heat mat is, it should be provided from above, and then only in a small area of the brooder. 98* is too warm. The entire brooder should NEVER be heated. I would remedy the heat situation and get some Poultry Nutri Drench.