if you're using vtf crosshairs try to look into tf_weapon_scattergun. Open it with notepad or the software of your choice and find this line. If "TracerEffect" is set to "", set it back to "bullet_scattergun_tracer01" and you should have your tracers back in no time =).
"DoInstantEjectBrass" "0"
"BrassModel" "models/weapons/shells/shell_shotgun.mdl"
"TracerEffect" "bullet_scattergun_tracer01"