need answers bout my sons bmx

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we've had an ongoing problem for a couple of months,his inner tube keeps going flat,i've replaced it a few times but still this happens,so i asked at the bike shop,they said to put a couple of layers of electrical tape on the inside of the tyre,and a couple of layers on the rim of the wheel,i did this and also replaced the inner tube AGAIN,,it was flat again within 30 minutes,,why is this happening coz it's really starting to annoy me now