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Hello @Lea1286, thanks for joining the Fitbit Community, it's great to see new faces around! 
Since when are you experiencing this situation? Do you have MobileTrack installed in the app? Have you restarted your tracker lately? Please keep in mind that factors such as tracker placement, dominant or non-dominant wrist, and stride length can all affect the number of steps recorded. Note that if you're pushing a stroller or shopping cart, you might try putting your wrist-based tracker in your pocket.
If after taking these factors into account you still think your tracker is significantly undercounting steps, please try a step test:
1. Put your wrist-based tracker on your wrist and check the step count at this point
2. Walk 100 steps, making sure to count a step each time one of your feet hits the ground.
3. Pause, then check your tracker to see your step count.
If your tracker counted less than 100 steps, let me know so I can check this further.
I hope this can be helpful, please give this a try and let me know the outcome!
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