Your Fitness Tracker detects your "steps" by using sensors to measure how your body is moving as you walk. It analyses what is going on (using secret methods) to try and work out when you are actually walking or running rather than just waving your arm about (if you have wrist based device).
Sometimes, when you carry out an activity, even though you are walking, your steps are not counted. Users have reported that pushing a pushchair or shopping trolley can reduce the number of steps that have been taken.
Its likely that this only affects wrist based devices and that since the arms are not free to move (i.e the arm is being used to push), the device is not picking up "normal" step measurements and so cannot adequately work out when a step is taken.
Other than pushing one handed, there is not much you can do to improve this situation. Some users have relocated their tracker to a pocket and have reported a high step rate but it may still not be accurate.
Since all manufacturers use their own methods to calculate steps, some may be influenced less by pushing a pushchair or shopping trolley than others.
We've put together a short video to try to show why things can go wrong.