No xp while fishing

Sometimes while fishing i will stop gaining xp. I am currectly at lvl 8 a little over halfway to lvl 9 and i have restarted minecraft 3 times but still i get no xp from fishing. I am using a fishing rod with mending and it only depletes its durability when this hppens. I don't know what's causing this to happen, just alot of times while fishing I stop gaining xp until i restart it a bunch.