Draw is resistance. Too tight and you are fighting the bunch; too loose and the cigar burns hot and fast. The cause is almost always construction: how the filler was folded (entubado or accordion), how hard it was molded, whether a vein or a lump is in the way.
Storage changes draw too. A cigar kept very wet swells and can feel tight. A dry cigar can feel easy and taste like paper. See relative humidity and humidor.
A cutter that crushes the head will ruin a good draw. So will biting the cap off. See cutter, guillotine, v cut, and punch.