There will be drain holes in the channel at each end on the outside and these may be blocked with cr@p. The sliding windows aren't waterproof like a fixed one, it relies on the water draining away through these slots.
It has not been unknown for some people to fit these sliding windows in the wrong way round leaving the drain holes on the inside

Hopefully yours just needs a clean out although the felt 'rope' may have disintergrated and causing the blockage