Make sure the resource have a schedule or work shift when the class or service is scheduled. If the resource have a schedule or work shift the time will be white instead of grey in Resource Planner (see the picture below)

Make sure the resource is added to the product´s resource requirements, either as an individual resource or through resource labels (Configuration > Products > Products > Resource requirements tab on the product)

If the resource is set up as point 1 and 2, check if the resource is already booked on another class/service.