If you are holding hand rails, that is why. Fitbit, on wrist, detects steps from the normal swinging of the arms when walking. When wrist is held still on hand rail, or when pushing shopping cart or baby carriage, steps might not get detected.
Your options:
1. Accept lower step count but know that calories are still good as they are based on heart rate.
2. If steps are more important to you, put tracker in pocket and probably get better step count as the expense of heart rate data.
3. If you can hold only one rail and let the other arm swing free, you get it all.
Before posting, re-read to see if it would make sense to someone else not looking at your Fitbit or phone.
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