Why does 1Password sometimes ask for my Master Password and not accept Touch ID

Note that 1Password (ONE password, get it?) should really be 2Passwords. You need to memorize your Master Password and be able to recall it at will AND you need to have your Secret Key written down somewhere (it is exceedingly difficult to memorize) as it will be needed as well from time to time. Touch ID is simply a temporary convenience that will disappear if and when your device, its operating system, the current version of 1Password, or your finger does.