The pump stopped working
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Viscosity is a pump specification
Pumps have an output and a viscosity range, and a lotion thicker than the pump was designed for will dose badly or not at all. Choosing a pump by neck finish alone is how a working product ends up in a package that cannot deliver it.
Dip tube length
Too short and the last of the product cannot be reached, too long and it kinks against the base. It is specified to the bottle height, and it is the thing that gets forgotten when a size changes and the closure does not.
Drying in the nozzle
Sunscreens and repellents can dry at the orifice between uses, particularly a product used twice a year rather than daily. A pump left through a winter frequently will not prime in spring, which arrives as a complaint at the start of the season.
Shipping without a lock
An unlocked pump in a box is a pump that has been pressed by other boxes, and the product arrives having leaked into the carton. A locking pump or a transit cap is the usual answer and it costs very little.
Test the actual combination
Fill the real bottle with the real product and use the pump through a whole container, including the last few doses. That catches viscosity, tube length and dose size together, and it is quicker than reasoning about any of them.
Have an answer for the customer
A pump that will not prime is recoverable with a few firm presses or by running warm water over the neck, and telling people that is better than replacing the unit. Printing it is better still.