125-200cc weak sixth gear

hello guys, i´ve got problem with my husqvarna CR 125 2000, i just bought it for good price so i knew i will have to repair something, so i immediately bought new piston, rod kit, bearings and seals, then i made it together, it now goes perfect except of sixth gear. it is very veak, it feels like it miss some gears between 5th and 6th, like there is some gap. when i ride it i´m going full gas every gear and every gear goes perfect, but when i place 6th, it wont go. does anyone know what can it be? it is the same if i am going uphill or flat road, it just wont go.. Thanks