Solar lights rely on sunlight to charge the internal battery. The more sunlight during the day the longer the light will operate at night. 

Ensure the solar light is positioned to get the maximum amount of sunlight during the day to maximise charging. If possible avoid shady areas.

During dull days or bad weather the solar light may not get a good charge so the lighting time at night may be drastically reduced. 

It is good to keep the solar panel clean to help maximise the power and battery charge. Also it may be good to switch off the solar light and allow a few days of charging before turning back on. this will fill the battery and allow for a longer light time.