A CT angiogram is more sensitive than a straight forward angiogram. They both rely on a radioactive substance being injected into your blood stream and it's this which the scanners pick up. If a CT angiogram doesn't reveal a vessel, then no way will an angiogram.
I had a vein that was closed after a bypass procedure and they looked for 30 minutes in a standard angiogram procedure to try and find it. At one point they believed they had, but a CT angiogram found the vein and it was in a totally different area.