The SLX pole sections are designed to be closed down onto the fixed end-cap so that when closed all sections have a small 5-10mm gap between each clamp. What may have happened is that some of the upper sections have been closed down fully before the bottom section was closed down and as a result the upper sections are preventing the base section from closing. To 'sort' this open up all of the clamp levers and slide the base section down first and then each section above it.
Alternatively if you did close the bottom section down first and it stopped there even with all the other sections extended then it is probably something trapped below the largest extending section and the end-cap. Slide the bottom extending section up a little and have a 'fish around' through the end-cap hole for whatever is jamming the section.