Why can't I withdraw from Coinbase?

I'm curious, why is it that I'm unable to withdraw my funds from Coinbase? I've been trying for the past few days, but the transaction keeps getting declined. Could there be any specific reasons for this? Is there a limit to the amount I can withdraw or perhaps a pending verification process that I'm unaware of? It's quite frustrating not being able to access my own money, so I'd appreciate any clarification you can provide. Thank you.