This is sadly not unusual for these things after some time of use. Depending on the quality of the product (battery, engine, etc). The battery often gets reduced over time, and eventually needs to be changed. I have no knowledge of this exact product, so it could also be a great quality, but faulty battery. In either case you would have to by a new battery.
Another possibility is heavy spinning wheels. Too much resistance quickly depletes the battery. Easy to check out by turning the front wheel, but the rear wheel is harder examined as it is connected to the engine. If you are unsure, you could remove the chain.
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