This is completely normal for a bicycle that's not run in. The brake linings in the drum are not worn in to the drum yet. Simply put; they haven't got to know each other yet. When the drum brake has been put to use for some time it will show its real mettle. The drum brake is powerful and easy to handle.
It you use the drum brake frequently you may need to adjust the brake wire. It's easy to do with a adjustable spanner or a ring spanner size 10. Please contact your bicycle dealer if you are uncertain how to proceed.