Most needle cartilage piercings are pierced with a 16 gauge (1.2mm) needle and fitted with 16g initial jewellery for healing, however if you had your ear pierced with a gun, it will have a with a much thinner post around 20 gauge.  The reason why you're having trouble is because your piercing hole simply isn't wide enough for the 16 gauge jewellery. If you were pierced with a 16g needle but removed the jewellery and started wearing thinner earrings, this will cause your piercing hole to shrink down to the size of the earring being worn. To find out more, read our guide here.