Seems to be a quite common issue with the signing of the build from what I can see. Us Linux bods had a few issues whereby no prompt was shown to enter the root password for sudo purposes for some reason or other, it escapes me, but this sounds similar as in that prompt that should be displayed for you to approve signing is not being shown...
Maybe you previously checked 'Always Allow'? Not sure if there is anyway you can undo such an action so the dialogue is displayed again?
Here are some search results for this issue that may help? I'm not an IOS expert nor an Apple User so don't know if this is in any way helpful, but maybe?