When you first create your BitGo account you choose a password that will give you access to your account, where you can change settings pertaining to your account and view your wallets. When you create a wallet, you have the choice to use the same password for your wallet as your login password, OR you can choose a different wallet password so that if you login password is compromised by an attacker, the attacker won't necessarily have access to your wallet. This wallet password will be used to change any settings on the wallet, and most importantly it's used to send bitcoin from your wallet.
If you change your login password your wallet passwords WILL NOT change. Updating your wallet passwords to the new login password would require us to know your existing wallet passwords, which we do not, and in general a user might not want to update some of their wallet passwords with this new login password.
This means if you're able to login but having trouble with your password when trying to send coin then you either chose a different wallet password than your login password to begin with, or you switched your login password and didn't update your wallet passwords.
If you've forgotten your wallet password, then please see this page of our FAQ