I can't make a custom shield in minecraft on my xbox

I'm having trouble making a custom shield in Minecraft on my xbox, I made one awhile back, but I added other things to the banner and I can't attach the banner to the shield anymore. I've tried putting less to nothing to the banner, but it still doesn't work and I don't know if this a bugged or maybe it I can't customize the shield at all. If anyone can get back to me and tell me how to fix this I would really appreciate it.

Thank you