As you're smoking it, any incompletely vaporized resins, partially-burned plant material, and even ash particles from the lit end do end up in the inside of the part of the joint that is not burning, and this can eventually clog the other end with undesirable tars- the same 'resin' that some people scrape off the inside of their glass pieces to smoke.
To keep this from happening, try rolling the mouth end a little fat, and a little looser, instead of tapering it down to a point like some people do.